Weird Noises from My Furnace: A Bay Area Homeowner's Guide to What That Sound Means

That Sound is NOT Normal

It’s 2 AM on a chilly November night in Walnut Creek. You wake up to a strange sound coming from your furnace. BANG. Then a metallic screech. Then silence.

Your heart races. Is it going to explode? Is my family safe? Should I call 911?

Most furnace noises aren’t emergencies—but some are. Knowing the difference could save your life.

This guide will help you identify what that sound means, whether it’s dangerous, and what to do about it.

Need help troubleshooting? Use our HVAC Diagnostic Tool for interactive troubleshooting with step-by-step repair recommendations.

⚠️ IMMEDIATE DANGER SIGNALS: Call 911

Before we discuss normal vs. concerning noises, these situations require immediate action:

Call 911 Immediately If:

1. You smell gas (rotten egg odor)

• Evacuate immediately
• Don’t touch light switches or create sparks
• Call 911 from outside your home
• Don’t return until cleared by fire department

2. Carbon monoxide alarm sounds

• Evacuate immediately
• Call 911
• Seek fresh air
• Get medical attention (CO poisoning symptoms: headache, dizziness, nausea)

3. You see flames outside the furnace

• Turn off furnace if safe to do so
• Evacuate
• Call 911

4. Smoke coming from furnace

• Turn off furnace at breaker
• Evacuate if smoke is heavy
• Call 911

For all other noises: Read this guide, then call an HVAC professional. (510) 391-5597

The 10 Common Furnace Sounds (Decoded)

1. BOOM or BANG (When Furnace Starts)

What it sounds like: A loud boom, thud, or bang when the furnace first ignites. May shake the house.

What’s happening: Gas builds up in the combustion chamber before igniting, causing a small explosion when it finally lights.

Causes:

• Delayed ignition (most common)
• Dirty burners
• Improper air-to-gas ratio
• Failed igniter (takes multiple attempts to light)
• Gas pressure issues

Danger level: 🔴 HIGH

Why it’s dangerous:

• Repeated mini-explosions can crack the heat exchanger
• Cracked heat exchanger = carbon monoxide leak into your home
• Each boom stresses metal components
• Can cause catastrophic failure

Bay Area consideration: If you rarely use your furnace (coastal areas), delayed ignition is more common when you finally turn it on. Never ignore booming sounds.

What to do:

• ⚠️ Turn off furnace immediately
• Don’t use until professionally inspected
• Call for service same day: (510) 391-5597
• Likely repair: Burner cleaning, igniter replacement ($200-$500)

Real example: San Mateo homeowner ignored booming for 2 weeks. Heat exchanger cracked. Family experienced headaches (CO exposure). Required emergency furnace replacement: $7,500. Early repair would have been $280.

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2. SCREECHING or SQUEALING (High-Pitched)

What it sounds like: Loud, high-pitched squealing or screeching. Like nails on a chalkboard or a car with bad brakes.

What’s happening: Metal rubbing on metal, usually from a blower motor bearing failure.

Causes:

• Worn blower motor bearings (most common)
• Failed belt (older furnaces with belt-driven blowers)
• Stuck or dry shaft bearings
• Lack of lubrication

Danger level: 🟡 MODERATE

Why it matters:

• Blower motor about to fail
• When motor seizes, no heat
• Can overheat and cause damage
• Might fail during coldest weather
• Fire risk if motor overheats

What to do:

• Turn off furnace to prevent motor seizure
• Call for service within 24-48 hours
• Don’t run continuously (causes more damage)
• Repair: Motor replacement $400-$800, bearing lubrication $150-$250

Temporary fix (if you must have heat):

• Can run furnace intermittently until repair
• Monitor closely
• Turn off if noise gets worse or smells like burning

Bay Area note: Peninsula and SF homes often have older furnaces with belt-driven blowers. Belt squealing is common but needs service.

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3. RATTLING or VIBRATING

What it sounds like: Metallic rattling, vibration, or loose parts shaking. May come and go with blower cycling.

What’s happening: Loose components vibrating against each other.

Causes:

• Loose access panel (most common and harmless)
• Loose blower wheel
• Unsecured ductwork
• Heat exchanger cracking (serious)
• Loose mounting bolts
• Debris in blower

Danger level: 🟡 MODERATE (depends on cause)

What to do:

DIY check first:

1. Turn off furnace
2. Open access panel
3. Look for obvious loose parts
4. Tighten panel screws
5. Turn furnace back on
6. If rattling stops: Fixed! (it was just the panel)
7. If rattling continues: Call professional

Professional needed if:

• Rattling inside furnace cabinet
• Rattling gets worse over time
• Accompanied by other symptoms
• Can’t identify source

Danger scenario: If rattling is coming from the heat exchanger area and accompanies a booming sound, this could indicate a cracking heat exchanger. Call immediately.

Typical repair cost: $80-$400 depending on cause

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4. SCRAPING (Metal-on-Metal)

What it sounds like: Harsh scraping or grinding, like metal dragging on metal. Sounds terrible.

What’s happening: Blower wheel contacting the housing or damaged blower assembly.

Causes:

• Blower wheel loose on shaft
• Broken blower wheel
• Bent fan blade
• Failed blower motor bearing causing wobble
• Foreign object in blower

Danger level: 🔴 HIGH (for the equipment, not life-threatening)

Why it’s serious:

• Can destroy blower wheel
• Can damage blower housing
• Creates metal shavings that spread through ductwork
• Will get progressively worse
• Eventually: Complete blower failure = no heat

What to do:

• ⚠️ Turn off furnace immediately
• Don’t run (causes more damage every minute)
• Call for emergency service
• Damage worsens rapidly
• Repair if caught early: $300-$600
• If blower destroyed: $800-$1,500

Don’t wait: Scraping means active destruction is happening.

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5. RUMBLING (After Furnace Shuts Off)

What it sounds like: Low rumbling or roaring that continues for 5-15 seconds after burners shut off.

What’s happening: Burners not shutting off cleanly, continuing to burn residual gas.

Causes:

• Dirty burners (most common)
• Improper air mixture
• Failed gas valve
• Delayed shutoff

Danger level: 🔴 HIGH

Why it’s dangerous:

• Indicates combustion issues
• Can lead to heat exchanger damage
• Carbon monoxide risk
• Gas valve may be failing
• Burners may not shut off completely

What to do:

• Note how long rumbling continues
• Schedule service within 48 hours
• If rumbling lasts >20 seconds: Turn off and call immediately
• Monitor for gas smell
• Repair: Burner cleaning and adjustment $200-$400

Bay Area consideration: Natural gas quality varies across PG&E service territory. Burner adjustments may be needed for your specific area’s gas composition.

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6. CLICKING or TICKING

What it sounds like: Rhythmic clicking, ticking, or tapping. May be rapid or slow.

What’s happening: Multiple possible causes, from harmless to serious.

Causes:

Harmless clicking:

• Ductwork expanding/contracting with temperature (normal)
• Gas valve clicking on/off (normal)
• Thermostat relay clicking (normal)

Concerning clicking:

• Failed igniter (rapid clicking, no ignition)
• Cracked heat exchanger (clicking during heating)
• Flame rollout (clicking with whooshing)
• Loose gas line connection (with gas smell = emergency)

Danger level: 🟢 LOW to 🔴 HIGH (depends on source)

What to do:

If clicking and furnace works normally: Likely harmless. Monitor.

If clicking and furnace won’t start: Failed igniter or ignition issue. Call for service.

If clicking with gas smell: ⚠️ Emergency. Turn off gas, evacuate, call 911.

If clicking only during heat cycle: Could be heat exchanger. Schedule inspection.

Typical repair: Igniter replacement $200-$350, ductwork adjustment free-$200

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7. WHISTLING or HISSING

What it sounds like: High-pitched whistle or hissing sound.

What’s happening: Air escaping through a small opening or being forced through a restriction.

Causes:

• Dirty air filter (most common) - restricted airflow
• Leaking ductwork
• Undersized return duct
• Partially closed damper
• Cracked heat exchanger (with combustion gases)
• Gas leak (extremely dangerous)

Danger level: 🟡 MODERATE to 🔴 HIGH

What to do:

DIY check:

1. Turn off furnace
2. Check air filter
3. Replace if dirty
4. Turn furnace back on
5. If whistle stops: Problem solved!

If whistling continues:

• Check all registers are open
• Look for closed dampers
• Listen for source

⚠️ If whistling + gas smell: Emergency. Evacuate and call 911.

If whistling + soot or yellow flame: Heat exchanger issue. Call immediately.

Typical repair: Filter change (DIY), duct sealing $400-$1,200, heat exchanger replacement $1,500-$3,500

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8. HUMMING (Loud or Unusual)

What it sounds like: Loud humming or buzzing, louder than normal operation.

What’s happening: Electrical component issues or transformer problems.

Causes:

• Failing blower motor (most common)
• Transformer issues
• Capacitor failure
• Electrical relay stuck
• Loose electrical connections
• Faulty inducer motor

Danger level: 🟡 MODERATE

Why it matters:

• Component about to fail
• Electrical fire risk
• Could fail in middle of cold night
• May damage other components

What to do:

• Safe to operate short-term
• Schedule service within 1 week
• Don’t ignore (will get worse)
• Turn off if humming gets louder or electrical burning smell
• Repair: Motor/capacitor $250-$800

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9. POPPING (From Ductwork)

What it sounds like: Loud popping or booming from ducts when furnace starts or stops. Sounds like someone hitting ductwork.

What’s happening: Ductwork expanding and contracting with temperature changes. Also called “oil-canning.”

Causes:

• Undersized ductwork (most common in Bay Area)
• Improperly supported ducts
• Flexible ducts (more prone to noise)
• Pressure imbalances
• Cheap thin-gauge metal ducts

Danger level: 🟢 LOW

Why it happens in Bay Area: Many homes have undersized return ducts, creating negative pressure that makes ducts “suck in” and pop.

What to do:

• Usually harmless (annoying, not dangerous)
• Can be fixed with duct modifications
• Stiffening ribs can be added
• Return duct sizing may need upgrade
• Not urgent unless very loud

Repair cost: $300-$1,500 depending on duct modifications needed

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10. NO SOUND (Furnace Won’t Start)

What it sounds like: Nothing. Complete silence when furnace should be running.

What’s happening: Power issue, safety lockout, or failed component.

Causes:

• Tripped circuit breaker (check first!)
• Blown fuse
• Failed thermostat
• Safety limit switch tripped
• Flame sensor dirty
• Dead transformer
• Failed control board

Danger level: 🟢 LOW (no heat, but usually not dangerous)

What to do:

DIY troubleshooting:

1. Check circuit breaker
2. Check thermostat batteries
3. Verify thermostat set to HEAT and temperature above room temp
4. Check furnace power switch (looks like light switch near furnace)
5. If all above check out: Call professional

Not DIY:

• Don’t bypass safety switches
• Don’t jumper connections
• Don’t force reset multiple times

Typical repair: $150-$600 depending on failed part

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🚨 THE MOST DANGEROUS SOUND: Cracked Heat Exchanger

What is a Heat Exchanger?

The heat exchanger is the most critical safety component in your furnace. It’s a metal chamber where:

• Gas burns inside the heat exchanger
• Combustion gases (including carbon monoxide) stay inside
• Air from your home flows AROUND the outside
• Heat transfers through the metal
• Clean, warm air goes to your home
• Deadly gases exit through the flue

When the heat exchanger cracks: Combustion gases mix with the air going to your home. This means carbon monoxide enters your living space.

Warning Signs of Cracked Heat Exchanger

Sounds:

• Booming on startup (stress cracks)
• Rattling from heat exchanger area
• Hissing during heat cycle
• Clicking when heating

Visual signs:

• Soot around registers
• Water dripping from furnace (condensation from combustion gases)
• Rust or corrosion on furnace
• Yellow or flickering flame (should be steady blue)
• Visible cracks (if you can see heat exchanger)

Health symptoms (MOST IMPORTANT):

• ⚠️ Headaches that go away when you leave home
• ⚠️ Dizziness or nausea
• ⚠️ Flu-like symptoms without fever
• ⚠️ Pets acting lethargic
• ⚠️ Carbon monoxide detector alarming

Why Heat Exchangers Crack

Common causes:

1. Age (15-20 years = high risk)
2. Delayed ignition (booming damages metal)
3. Improper sizing (oversized furnace = short cycling = stress)
4. Dirty filters (restrict airflow = overheating)
5. Lack of maintenance (corrosion weakens metal)
6. Poor installation (airflow restrictions)

Bay Area factor: Coastal homes with high humidity and infrequent furnace use can develop corrosion that weakens heat exchangers faster than expected.

What to Do If You Suspect a Crack

Immediate actions:

1. ⚠️ Turn off furnace at thermostat
2. Turn off furnace at circuit breaker
3. Open windows for fresh air
4. Leave home if anyone has symptoms
5. Call for emergency inspection: (510) 391-5597

Don’t:

• ❌ Keep using furnace “just one more day”
• ❌ Only turn it on at night
• ❌ Try to repair yourself
• ❌ Ignore symptoms

Inspection includes:

• Visual inspection with camera
• Combustion analysis
• Carbon monoxide testing
• Pressure testing
• Complete safety check

If cracked: Furnace must be replaced. Heat exchangers usually can’t be economically repaired.

Cost: $1,500-$3,500 for replacement, or $4,000-$8,000 for new furnace

Your life is worth more than the repair cost.

Carbon Monoxide: The Silent Killer

Why it’s so dangerous:

• Odorless
• Colorless
• Tasteless
• Symptoms feel like flu
• Can kill while you sleep
• Permanent brain damage possible

Symptoms by exposure level:

Low exposure (50-70 ppm):

• Mild headache
• Fatigue
• Nausea

Moderate exposure (150-200 ppm):

• Severe headache
• Dizziness
• Confusion
• Nausea/vomiting

High exposure (400+ ppm):

• Loss of consciousness
• Death within hours

Critical exposure (800+ ppm):

• Convulsions
• Death within minutes

Protect yourself:

• Install CO detectors on every floor
• Test detectors monthly
• Replace every 5-7 years
• Annual furnace inspection
• Never ignore symptoms

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Bay Area-Specific Furnace Issues

1. Coastal Corrosion (San Francisco, Pacifica, Half Moon Bay)

Problem: Salt air accelerates heat exchanger corrosion

Sounds:

• Earlier heat exchanger failure than expected
• Hissing from corrosion holes
• Rumbling from damaged burners

Solution:

• Annual inspections critical
• Consider corrosion-resistant models
• Proper ventilation prevents moisture buildup

2. Infrequent Use (Mild Climates)

Problem: Furnace sits idle 8-9 months per year

Sounds:

• Booming on first use (delayed ignition)
• Squealing from dry bearings
• Rattling from settled components

Solution:

• Run furnace monthly even in summer (prevents seizing)
• Pre-season inspection before winter
• Lubricate components during tune-up

3. Old Homes with Original Furnaces (Oakland, Berkeley, SF)

Problem: Many Bay Area homes have 30-50 year old furnaces

Sounds:

• All the sounds (old age = multiple issues)
• Particularly: Rumbling, squealing, booming

Solution:

• Consider replacement
• Modern furnaces 40% more efficient
• Safety features non-existent in old models
• Replacement often better investment than repair

4. Earthquake Aftermath

Problem: Seismic activity can shift/damage furnaces

Sounds:

• New rattling after earthquake
• Hissing (gas connections loosened)
• Scraping (blower shifted)

Solution:

• Inspect after any significant earthquake
• Check gas connections
• Verify mounting bolts tight
• Test for gas leaks

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Your Furnace Noise Action Plan

Sound Classification Quick Reference

🔴 CALL IMMEDIATELY (Same Day):

• Booming/banging on startup
• Scraping metal-on-metal
• Rumbling after shutoff (>10 seconds)
• Hissing with gas smell
• Any sound + health symptoms

🟡 CALL WITHIN 48 HOURS:

• Squealing/screeching
• Rattling (internal)
• Loud humming
• Clicking (furnace won’t start)
• Rumbling after shutoff (<10 seconds)

🟢 MONITOR/SCHEDULE SERVICE:

• Ductwork popping
• Soft whistling (check filter first)
• Normal clicking
• Quiet rattling (loose panel)

DIY Checks Before Calling

Safe to check yourself:

1. ✅ Air filter (change if dirty)
2. ✅ Thermostat settings and batteries
3. ✅ Circuit breaker
4. ✅ Furnace power switch (near unit)
5. ✅ Access panel (tighten if loose)
6. ✅ Registers (make sure all open)

DON’T attempt:

1. ❌ Opening combustion chamber
2. ❌ Adjusting burners or gas valve
3. ❌ Bypassing safety switches
4. ❌ Repairing electrical components
5. ❌ Welding/patching heat exchanger
6. ❌ Cleaning inside furnace cabinet

When to Call Bay Area Climate Control

Emergency (24/7 service): (510) 391-5597

• Gas smell
• Carbon monoxide detector alarming
• Booming sounds
• Scraping sounds
• Health symptoms

Same-day service:

• Squealing
• No heat
• Loud unusual sounds

Scheduled service:

• Annual maintenance
• Pre-season inspection
• Minor rattling
• Performance issues

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Prevention: Stop Noises Before They Start

Annual Maintenance ($150-$250)

What we check:

• Clean and inspect burners
• Test ignition system
• Check heat exchanger for cracks
• Lubricate blower motor
• Test safety controls
• Measure gas pressure
• Check electrical connections
• Clean flame sensor
• Test thermostat
• Inspect venting
• Carbon monoxide test

Prevents:

• 90% of booming sounds (clean ignition)
• Squealing (bearing lubrication)
• Rumbling (burner adjustment)
• Early heat exchanger failure
• Emergency breakdowns

ROI: $200 maintenance prevents $2,000 average emergency repair

Regular Filter Changes

Schedule:

• 1” filters: Every 30-60 days
• 4” media filters: Every 6-12 months
• During wildfire season: Check monthly

Prevents:

• Whistling
• Overheating (heat exchanger damage)
• Blower motor strain
• System inefficiency

Cost: $5-$30 per filter vs. $400-$800 blower motor replacement

Keep Area Clear

• Don’t store items against furnace
• Keep 3 feet clearance
• Don’t block air intake
• Don’t block exhaust (for high-efficiency furnaces)

Prevents:

• Fire hazards
• Combustion issues
• Overheating
• Safety lockouts

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Real Bay Area Examples

Case 1: The Ignored Boom (Walnut Creek)

Situation:

• Homeowner heard booming for 3 weeks
• “It still works, so I’ll wait”
• Developed headaches, blamed stress

Outcome:

• Heat exchanger cracked
• CO levels 85 ppm (moderate poisoning)
• Emergency furnace replacement: $6,800
• ER visit for CO exposure: $2,400
• Total cost: $9,200

If called at first boom:

• Burner cleaning: $250
• Heat exchanger intact
• Family safe

Lesson: Booming is never normal. Call immediately.

Case 2: The DIY Disaster (San Jose)

Situation:

• Squealing blower motor
• Homeowner watched YouTube video
• Attempted to lubricate motor
• Broke wire during reassembly

Outcome:

• Motor replacement: $650
• Control board damaged: $800
• Service call: $150
• Total: $1,600

If called professional initially:

• Bearing lubrication: $200
• Motor saved

Lesson: Some repairs need professionals.

Case 3: The Lucky Catch (Redwood City)

Situation:

• Slight rattling noticed
• Called for inspection
• “Probably just loose panel, but let’s check”

Outcome:

• Heat exchanger had hairline crack (early stage)
• No CO leak yet, but would have within weeks
• Replacement covered by warranty
• Cost: $0

If waited:

• Warranty would have expired in 2 months
• Out-of-pocket replacement: $3,200

Lesson: Early detection saves money and lives.

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Frequently Asked Questions

Q: My furnace has been making a sound for years. Is it still dangerous?
Long-standing noises can become dangerous. Metal fatigue worsens over time. Have it inspected—what was harmless 5 years ago may be critical now.

Q: Can I just turn up the volume on my TV to ignore the noise?
Furnace noises are warning signs, like your car’s check engine light. Ignoring them leads to expensive repairs or safety hazards.

Q: How often should I have my furnace inspected?
Annually, ideally in fall before heating season. Bay Area coastal homes: Consider twice-yearly due to corrosion issues.

Q: My furnace is only 5 years old. Why is it making noise?
Age isn’t everything. Poor installation, lack of maintenance, or manufacturing defects can cause early problems.

Q: Is it normal for my furnace to make ANY noise?
Some noise is normal: Gentle air whoosh, soft click of gas valve, quiet hum of blower. Loud, harsh, or new sounds are not normal.

Q: Can a cracked heat exchanger be repaired?
Rarely. Heat exchangers operate under high heat and pressure. Welding is temporary at best and often illegal. Replacement is the safe solution.

Q: Should I buy a carbon monoxide detector?
Yes! Required by California law in all homes. Install on every floor, especially near bedrooms. Test monthly. Replace every 5-7 years.

Q: What if the noise only happens sometimes?
Intermittent noises can indicate early-stage problems. Schedule inspection before it becomes constant (and more expensive).

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Don’t Wait Until It’s Too Late

Furnace noises are warning signs. Your furnace is trying to tell you something is wrong.

• ✅ Catch problems early = smaller repairs
• ✅ Prevent heat exchanger cracks = save lives
• ✅ Avoid emergency breakdowns = save money
• ✅ Maintain efficiency = lower bills

Bay Area Climate Control
Carrier Factory Authorized Dealer
Serving the Bay Area Since 2010

Phone: (510) 391-5597
Emergency: 24/7 Available
Online: Request Service

Free Furnace Noise Diagnosis

Describe the sound when you call. We’ll tell you:

• Urgency level
• Likely cause
• Expected repair cost
• Whether it’s safe to operate
• Same-day service if needed

Service Area:
Alameda • Contra Costa • San Francisco • San Mateo • Santa Clara Counties

Your safety is our priority. That sound won’t fix itself.