What's That Noise? Decoding Strange HVAC Sounds

Your HVAC Speaks - Learn Its Language

Strange sounds from your heating or cooling system aren't just annoying - they're warning signs. Learning to interpret these sounds can help you catch problems early and know when to call for help.

Banging or Clanking

From Your Furnace

Possible Causes:

• Delayed ignition (gas builds up, then ignites with a bang)
• Cracked heat exchanger (SERIOUS - CO risk)
• Loose blower wheel
• Expanding/contracting ductwork

Urgency Level: HIGH - especially if accompanied by gas smell

What to Do:

• If you smell gas, leave house and call gas company
• If no gas smell, turn off system and call for service
• Don't ignore - delayed ignition can crack heat exchanger

From Your AC

Possible Causes:

• Loose fan blade hitting something
• Compressor mounting issues
• Debris in outdoor unit

Urgency Level: MEDIUM - won't cause immediate damage but indicates problem

What to Do:

• Turn off AC
• Check outdoor unit for visible debris
• Call for service if noise persists

Squealing or Screeching

High-Pitched Squeal on Startup

Possible Causes:

• Worn blower belt (older systems)
• Dry motor bearings
• Blower motor issue

Urgency Level: MEDIUM - belt could snap, leaving you without heat/cool

What to Do:

• If belt-driven system: belt likely needs replacement
• If direct-drive system: motor bearings need lubrication or motor replacement

Continuous Screech

Possible Causes:

• Metal-on-metal contact
• Bearing failure imminent

Urgency Level: HIGH - turn off immediately to prevent further damage

Rattling

Light Rattling

Possible Causes:

• Loose screws or panels
• Debris in ducts or unit
• Register covers loose

Urgency Level: LOW - annoying but not usually serious

What to Do:

• Tighten visible screws
• Check register covers
• Remove debris if accessible

Heavy Rattling

Possible Causes:

• Loose blower assembly
• Motor mount issues
• Compressor problems (AC)

Urgency Level: MEDIUM - should be addressed soon

Clicking

Clicking at Startup (Normal)

Relays and contactors clicking when system starts is normal.

Repeated Clicking Without Startup

Possible Causes:

• Failed thermostat relay
• Defective control board
• Loose wiring
• Failing capacitor

Urgency Level: MEDIUM-HIGH - system won't work properly

What to Do:

• Check thermostat batteries
• If batteries good, call for electrical diagnosis

Clicking From Outdoor Unit

Possible Causes:

• Relay clicking repeatedly (trying to start)
• Capacitor failing
• Compressor issues

Urgency Level: HIGH - continued attempts can burn out compressor motor

Hissing

From Indoor Unit

Possible Causes:

• Refrigerant leak (most concerning)
• Leaky ductwork
• Normal expansion valve operation (brief)

Urgency Level: MEDIUM-HIGH if continuous

What to Do:

• If accompanied by reduced cooling, likely refrigerant leak
• Check for ice on refrigerant lines (indicates leak)
• Call for service - refrigerant requires professional handling

From Outdoor Unit

Possible Causes:

• High-pressure refrigerant leak
• Normal operation during defrost cycle (heat pumps)

Urgency Level: HIGH if continuous

Humming or Buzzing

Low Continuous Hum

Usually normal operation - transformers and motors produce slight hum.

Loud Buzzing

Possible Causes:

• Electrical issues (loose connections, failing contactors)
• Compressor problems
• Capacitor failure

Urgency Level: MEDIUM-HIGH - electrical issues can be fire hazards

What to Do:

• If from outdoor unit while AC won't start: likely capacitor
• Call for electrical diagnosis

Gurgling or Bubbling

Possible Causes:

• Refrigerant line issues
• Condensate drain problems
• Low refrigerant charge

Urgency Level: MEDIUM

What to Do:

• Check if condensate drain is clogged
• Gurgling in refrigerant lines suggests low charge (leak)

Whistling

Possible Causes:

• Dirty air filter (most common)
• Blocked return vents
• Ductwork too small
• Air leaks in ducts

Urgency Level: LOW-MEDIUM

What to Do:

• Check and replace air filter
• Ensure return vents aren't blocked by furniture
• If persistent, have ductwork evaluated

When to Call Immediately

Call right away if you hear:

• Loud banging from furnace (especially with gas smell)
• Metal-on-metal screeching
• Repeated clicking without system starting
• Continuous hissing (possible refrigerant leak)

The Bottom Line

Your HVAC system shouldn't sound like a symphony of strange noises. While some sounds are normal, unusual or new sounds warrant attention. When in doubt, it's better to have a technician check it out than to let a small problem become an expensive repair.