AC Making a Loud Noise? What Each Sound Means (Buzz, Rattle, Squeal, Bang)

An air conditioner makes some noise — the outdoor fan, the indoor blower, a click when it starts and stops. A new noise, or a louder one, is the system telling you where to look. Match the sound.

Safety first, every page: if you smell gas anywhere in the house, leave and call your gas company from outside.

Buzzing

• Outdoor unit buzzes, fan not spinning: shut it off now. That's a fan motor or capacitor problem — see AC outdoor fan not spinning.
• Outdoor unit buzzes and runs normally: could be a loose panel or a part vibrating. Turn power off and check that the panels are screwed down and nothing's leaning on the unit.
• A loud buzz and then nothing: often a contactor or capacitor. Pro.

Rattling

• Outdoor unit: leaves, twigs, or a stone in the fan compartment, or a loose panel. Turn power off (breaker or shutoff box beside the unit), look through the top grille, remove only what you can see and grab. Check that the top grille and side panels are tight.
• Indoor unit or ducts: a loose filter cover or a filter that's the wrong size, rattling in its slot. Check the filter is seated and the cover is latched.
• Rattle that turns into a clank: something's about to come apart. Shut off, call.

Squealing or screeching

A high-pitched squeal at startup that fades — often a fan motor bearing on its way out, or a belt on older indoor blowers. A squeal that doesn't stop, or gets louder, means shut it off; a motor that seizes takes other parts with it.

Banging or clanking

• Outdoor unit, on startup: hard-starting compressor or a loose mount. Pro.
• Indoor, a "boom" when the blower starts: flexible ductwork or a metal plenum flexing under pressure — annoying, usually harmless, sometimes helped by opening a closed vent.
• Repeated banging while running: shut it off. Something is hitting something.

Hissing

• A brief hiss when the system shuts off: pressure equalizing. Normal.
• A continuous hiss or bubbling from the lines or the indoor unit: a refrigerant leak. Shut it off, call. Running low on refrigerant damages the compressor.

Clicking

• One click at start and stop: normal — relays and the thermostat.
• Rapid clicking, system not starting: a control or capacitor issue. See AC won't turn on.

Before you call

Run the free troubleshooter. It captures which unit, which sound, when it happens, and what you checked, and gives you a summary to read to the dispatcher — "outdoor unit, loud buzz, fan not turning" gets the right part on the truck.

Related symptoms

• AC outdoor fan not spinning
• AC won't turn on
• Furnace making a weird noise

Start the free troubleshooter → Free. No sign-up. Don't pay $150 to flip a switch.

Free · no sign-up · no personal info · about ten minutes