What the noise usually is
- A humming or buzzing outdoor unit that will not start. Classic failing capacitor. The motor is trying to start without the electrical kick it needs.
- A rapid chattering or clicking. Often the contactor, whose contacts have pitted and are no longer making cleanly.
- An electrical buzz that rises and falls with the fan. Worth investigating rather than living with.
- Vibration buzz. Sometimes simply a loose panel or a fan blade catching, which is minor but should not be ignored.
Noises that are more urgent
A loud metallic grinding or screech suggests bearings or a blade striking something. Switch the system off and get it looked at rather than running it.
A hissing or bubbling sound is a different signal and can indicate refrigerant escaping.
Why not to investigate it yourself
The buzz is usually electrical, and the components producing it are the ones that hold a charge after the power is off. A capacitor can discharge into you well after the breaker is off.
What to do
Note when it happens: at startup, continuously, or only in certain modes. That detail genuinely helps. Then have it looked at. A capacitor is an ordinary repair, and catching it early avoids the motor wearing itself out trying to start.
Common questions
Why does my AC hum but not start?
Most often a failed capacitor. The motor is trying to start without the electrical assistance it needs, which is why it hums rather than turning.
Is a buzzing AC dangerous?
The noise itself is usually an electrical component failing rather than an immediate hazard, but it should not be left. Opening the unit to look is the dangerous part.
What noises mean I should switch it off?
Loud metallic grinding or screeching, and any hissing or bubbling that could indicate escaping refrigerant.
How much is a capacitor replacement?
An owner supplied example figure is quoted before the work starts. Confirm inclusions with us. It starts with a $139 diagnostic, waived when you book the repair.
Reviewed and updated September 2026. Prices shown are owner supplied typical ranges, confirmed after an assessment. If anything here looks out of date, call (615) 400-4993 and tell us.