What Different AC Noises Actually Mean

Every air conditioner makes some noise. What matters is whether the sound is new, and what kind of sound it is. Here's what the common ones usually indicate.

Rattling or vibrating

Most often a loose panel, a loose screw, or debris that has found its way into the outdoor unit. It's the least alarming noise on this list and frequently the cheapest to resolve. If it started right after a windy or dusty spell, debris in the condenser is the first thing to check.

Squealing or screeching

Usually a fan motor bearing or a belt on older commercial units. Worth attending to before it becomes a seized motor, which is a considerably larger repair.

Clicking that doesn't stop

A single click when the unit starts or stops is normal relay operation. Repeated clicking without the system starting suggests an electrical control fault, often a failing capacitor or contactor. This one is worth acting on promptly, because a unit that keeps trying and failing to start puts strain on the compressor.

Gurgling or bubbling

Typically refrigerant moving through the lines, which can be normal, or a blocked condensate drain, which is not. If it comes with any dampness, treat it as a drainage problem.

Banging or clanking

This is the one to take seriously. A hard mechanical bang usually means a loose or broken component inside the compressor. Switch the unit off and leave it off. Continuing to run it can turn a repairable fault into a full compressor replacement.

A humming outdoor unit with a stationary fan

The unit is drawing power but the fan isn't turning: commonly a failed capacitor. Switch it off rather than letting it sit there humming, since the compressor can overheat.

The general rule

Rattles and squeals are usually about wear, and can wait a short while. Banging, repeated clicking, and humming-without-spinning are electrical or mechanical faults where continuing to run the unit makes the eventual repair bigger. When in doubt, switching it off costs you a warm evening; running it through a mechanical fault can cost you a compressor.