Noise is the most useful diagnostic clue an AC gives you, because each fault has a characteristic sound.
Match the sound to the cause
Rattling. Loose panel, bent fan blade or debris in the blower housing. Often cheap to fix, but it worsens fast if a blade is out of balance.
Buzzing or humming with no airflow. Electrical. Usually a failing capacitor or contactor. Switch the system off — running it in this state can burn out the motor.
High-pitched screech on start-up. Bearing or motor wear in the indoor blower or outdoor fan.
Gurgling or bubbling. Condensate drain, or refrigerant flow. If it comes with weak cooling, suspect a low charge and a leak.
Clicking that repeats without the unit starting. A relay trying and failing to close.
Banging from a ducted system when it starts. Ductwork expanding, or a loose damper. Annoying rather than dangerous.
What to do before the visit
Record the sound on your phone from the indoor and outdoor units separately, note whether it happens at start-up or continuously, and note whether cooling is affected. A ten-second clip regularly saves a technician twenty minutes of diagnosis.
What not to do
Do not spray lubricant into a blower housing, and do not run a unit that buzzes without spinning. Both turn a small part replacement into a motor replacement.
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