A well-maintained AC should run at a fairly consistent, low hum. When a new or different sound appears, it's usually the unit telling you something specific is wrong — the type of sound is actually a useful diagnostic clue.
A rattling or buzzing sound
Usually caused by a loose panel, screw, or mounting bracket vibrating against the casing. This is one of the easier fixes — tightening the loose part usually resolves it completely.
A squealing or screeching sound
Often points to a worn fan motor bearing, either in the indoor or outdoor unit. Left unaddressed, a failing bearing eventually causes the motor to seize entirely, turning a cheap bearing replacement into a full motor replacement.
A clicking sound at startup or shutdown
A single click when the compressor engages or disengages is usually normal — that's the relay switching. Repeated or rapid clicking, however, can indicate an electrical relay issue that's worth having checked.
A hissing or whistling sound
This can indicate refrigerant escaping through a leak, especially if it's accompanied by reduced cooling. This isn't something to wait on — refrigerant leaks tend to worsen over time and affect the compressor if ignored.
A dripping or gurgling sound
Usually related to water movement in the drain line, and often paired with the AC leaking symptoms we cover in our article on AC water leaks.
When to stop diagnosing and call a technician
If the noise is new, getting louder, or paired with reduced cooling performance, it's time for a proper inspection rather than waiting to see if it goes away on its own — most noise-related issues get more expensive to fix the longer they're left running.