Guide

It is making a noise it did not used to make.

Each sound maps to a different part. Knowing which is which saves you a diagnosis.

Updated October 3, 2026EPA 608 certified

Air conditioners make noise. What matters is a noise that is new.

Buzzing or humming

From the outdoor unit, with nothing spinning. The signature of a failed capacitor. A motor is being asked to start, cannot, and sits there energised. Flat price on the list, and worth dealing with promptly because the current draw is high.

From the electrical compartment, rapid and chattering. A contactor pulling in and dropping out repeatedly, usually from pitted contacts or a weak control signal.

A low buzz with everything running normally. Often just the contactor coil doing its job. If nothing else changed, it is probably nothing.

Screeching or squealing

In older equipment this is bearings or a belt. A blower motor with failing bearings will squeal and then get louder over weeks.

A loud metallic screech is worth shutting the system off for. Bearings that seize while the motor keeps trying take the motor with them.

Clicking

Once at startup and once at shutdown is normal. That is the contactor.

Repeated clicking with no start is the control circuit trying and something refusing. Often a capacitor, sometimes a safety opening.

Clicking from the indoor unit can be a relay on the control board, or something as mundane as a flap of ducting.

Rattling

Mostly undramatic. Panel screws back out with vibration, and a Valley monsoon deposits a remarkable amount of gravel and plant matter inside an outdoor cabinet.

Worth distinguishing: a rattle from inside the fan shroud, where something loose can reach the blades, is worth stopping for. A rattle from a loose panel is a screwdriver.

Banging or clanking

A hard bang at shutdown can be a compressor with worn mounts. Rhythmic clanking while running suggests something contacting something else inside the cabinet. Both are shut-it-off noises.

Gurgling or hissing

Gurgling is often the condensate drain, and can mean a partial plug. Worth checking the drain pan before it finishes plugging.

Hissing that is new and persistent can mean a refrigerant leak. A sealed system does not consume refrigerant, so if it is going somewhere, there is a hole.

What it costs to settle

A noise narrows a diagnosis considerably, which is why it is worth being specific on the phone: where it comes from, when it happens, and whether it is new. Often it can be identified before anybody drives out.

If it needs a visit it is a diagnostic, and the repair price comes off the published list once the fault is known.

Straight answers

Questions that come with this one.

Which noises mean shut it off now?

A loud metallic grinding or screeching, anything that sounds like metal hitting metal, and any noise accompanied by a burning smell. Also a hard mechanical bang at shutdown. Those are worth stopping for. A hum, a click, or a rattle will usually wait until morning.

The outdoor unit buzzes but nothing spins. What is that?

Classic failed capacitor. A motor energised but unable to start hums rather than turns. It is also drawing far more current than normal while it does that, so do not leave it trying.

Is a rattling sound serious?

Usually not. Loose panel screws and debris in the cabinet account for most of it, and a Valley monsoon puts a surprising amount of gravel inside an outdoor unit. It is worth looking at, because a rattle from inside the fan shroud is a different story.

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