This one needs a licensed pro
Likely cause: a failing motor or electrical part
Loud buzzing, loud humming or clicking that keeps repeating usually traces to a motor or an electrical part. Turn the system off and call an HVAC technician.
Why this happens
Trane says a loud hum from the indoor unit could be a problem with the blower motor, and from the outdoor unit it could be the capacitor, the contactor or the fan motor. Bryant lists buzzing among the sounds that call for turning the system off and contacting a qualified HVAC professional, and Carrier lists a buzzing sound from the motor as a sign of a burnt out condenser fan motor. Trane says clicking can come from a capacitor or contactor outside, or from a control board, a loose connection or a safety switch inside. These parts carry or store electricity, so they are a technician's job.
What to do right now
- Set the thermostat to Off. Bryant says to turn the system off when you hear buzzing, to prevent further damage.
- If the outdoor unit hums but its fan isn't turning, leave it off, and don't push the fan through the grille to get it going. Carrier says running the AC without a working condenser fan can overheat the compressor.
- Keep the panels on both units closed. Trane says not to try replacing a run capacitor yourself, because it holds an electrical charge.
- Note when the noise happens: at startup, at shutdown, or the whole time the system runs. Trane says the more you can tell the technician, the better.
- Call an HVAC technician.
When to stop. If you see smoke or flames, the American Red Cross says to get out, stay out and call 911. If you smell burning plastic or an electrical burning smell at either unit, keep the system off. Trane says to shut off your HVAC system and call for repairs. If the AC's breaker trips, don't keep resetting it. Carrier says that if the system continues to trip the breaker, shut it down and call a professional HVAC technician.
What to tell the HVAC technician
- Whether it is a buzz, a hum or a click, and which unit it comes from.
- Whether the outdoor fan was spinning while you heard it.
- When it happens: at startup, at shutdown or all the time.
- Whether a breaker has tripped.
- The brand and model from the outdoor unit's data plate, or from the label on the outside of the indoor unit. Don't open a panel to find it.
Build the full list: what to tell the HVAC technician.
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