Strange Noises from Your HVAC System: What They Mean

Your HVAC system communicates through sound. Learning to interpret these sounds helps you catch problems early and know when to call for service.

Normal Sounds

A gentle humming from the outdoor unit when the AC runs. A soft whoosh of air from vents when the blower starts. Occasional clicking when the system starts or stops as relays and contactors engage. These are normal operating sounds.

Sounds That Need Attention

Squealing or screeching from the blower area usually indicates a worn belt on belt-driven systems or a failing bearing on direct-drive blowers. This will not fix itself and will get worse. Rattling from the outdoor unit may mean a loose panel, debris in the unit, or a failing fan motor mount. Buzzing from the outdoor unit can indicate a failing contactor, capacitor, or loose wiring. Hissing from refrigerant lines suggests a refrigerant leak that needs immediate attention.

Sounds That Need Immediate Action

Banging or clanking from the air handler may mean a broken blower wheel or disconnected component. Turn the system off and call for service. Grinding indicates metal-on-metal contact from a failed bearing. Continued operation will cause more damage. Repeated clicking without the system starting suggests a failing contactor or control board. A screeching outdoor unit may have a seized compressor or fan motor.

Boom on Furnace Startup

A loud boom when your gas furnace starts usually indicates delayed ignition -- gas builds up in the combustion chamber before igniting. This can be caused by a dirty burner, a failing igniter, or a gas valve issue. It is not dangerous at low levels but should be addressed before it damages the heat exchanger.

If your HVAC system is making unusual sounds, call PCG Climate for diagnosis. Early attention to unusual sounds prevents expensive failures.