Air Conditioning

Common HVAC Noises and What They May Mean

Most HVAC noises fall into four groups: electrical humming from a failing capacitor or contactor, mechanical grinding or screeching from bearings and motors, air noise from restricted airflow, and expansion sounds from ductwork. A hard hum with no start and a loud bang at furnace ignition are the two that need prompt attention.

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Sound by sound

Loud hum with no start: usually a failed run capacitor or a seized motor. Shut the system off to avoid overheating the compressor
Repeated clicking: a control relay trying to start a component that will not start
Grinding: motor bearings failing. The motor is near the end
Screeching or squealing: belt or motor bearing on older equipment
Rattling: loose panel, debris in the outdoor unit, or a failing fan blade
Banging at furnace startup: delayed ignition. Have it corrected promptly
Whooshing or a soft clunk on a heat pump: the reversing valve shifting, which is normal
Whistling at vents or the filter: restricted airflow, often an undersized filter or return
Popping in ducts: normal expansion and contraction of sheet metal

Which noises mean shut it off now

A hard hum with no motor start
Any burning or electrical smell
A loud bang each time the furnace lights
Grinding that gets louder while the system runs

Frequently asked questions

Is a clicking sound at startup normal?

A single click as the thermostat or contactor engages is normal. Repeated clicking without the system starting means a component is failing to start and should be diagnosed.

AC Repair

Diagnosis for unusual noise

Furnace Repair

Ignition and combustion noise

Heat Pump Repair

Reversing valve and defrost sounds

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