An air conditioner that starts squealing, grinding, banging, buzzing or rattling is usually pointing at one of four things: a loose part or debris, a motor with worn bearings, an electrical part straining to start a motor, or, less often, the compressor or the refrigerant circuit. Two questions narrow it down quickly: where is the sound, and when does it happen? A few noises mean the system should be switched off now. Most can wait for a scheduled visit.
Where the sound comes from
Stand next to the outdoor unit while the system runs, then next to the indoor cabinet, then by a few vents.
- Outdoor unit (condenser). It holds the compressor, the outdoor fan and its motor, the capacitor and the contactor. Clicking, humming, buzzing, banging and fan noise usually live here.
- Indoor cabinet. In a typical split system the indoor half is the furnace or air handler, with the cooling coil mounted on it and one blower serving both heating and cooling. A squeal or rumble here in the cooling season is usually the blower motor or blower wheel, so our page on furnace noises applies as well.
- Vents and walls. Whistling, popping or booming in the ducts points to airflow and ductwork rather than to the equipment itself.
When it happens
- At startup. A single click is normal. Repeated clicking, a hum that lasts several seconds before anything turns, or a squeal that fades as the motor comes up to speed are electrical or bearing issues.
- While running. Steady rattles, grinding, a constant screech, or a buzz that rises and falls.
- At shutdown. A brief hiss or gurgle as pressures settle is often normal. A hard clunk or a shudder can be a worn compressor mount or a refrigerant line knocking against the house.
Sound, likely cause and what it takes
| What you hear | Likely cause | What it usually takes |
|---|---|---|
| Squeal at startup that fades | Loose or glazed belt on an older belt-drive blower, or motor bearings starting to wear | Belt adjustment or replacement; bearing check |
| Screech that never settles | Fan or blower motor bearings failing | Motor replacement in most cases |
| Grinding or scraping | Worn bearings, or a fan blade or blower wheel rubbing its housing | Switch off; motor, blade or wheel repair |
| Banging or clanking | Loose or broken fan blade, loose blower wheel, or a compressor fault | Switch off; inspection decides which |
| Rattling or vibration | Loose panels or screws, debris in the fan, a refrigerant line touching a wall | Tightening, clearing debris, securing the line |
| Hum or buzz, outdoor fan not turning | Failed capacitor, sticking contactor or seized fan motor | Electrical testing and the failed part |
| Repeated clicking, nothing starts | Contactor, relay, capacitor or thermostat fault | Electrical diagnosis |
| Hissing or bubbling while running | Refrigerant leak | Leak search and repair |
What you can safely check
- Record it. Take a short video with sound and write down when the noise happens: at startup, all the time, or at shutdown.
- Cut the power before touching anything. Set the thermostat to Off, then pull the outdoor disconnect or switch off its breaker.
- Clear the outdoor unit. Remove leaves, twigs, paper and plastic from the top grille and the sides, and cut plants back so air can move freely.
- Tighten the outer panel screws you can reach on the outside of the cabinet. Leave the electrical compartment closed: a capacitor can hold a charge after the power is off.
- Look at the filter. A clogged filter makes the blower strain and can make return grilles whistle.
Do not spray lubricant into the unit, poke at the fan, or keep restarting a system that bangs or grinds.
Switch it off and call when you hear
- grinding or metal-on-metal scraping;
- screeching that does not settle down after startup;
- banging or knocking from the compressor area of the outdoor unit;
- loud electrical buzzing, crackling, or a burning smell;
- any noise that comes with weaker cooling, a tripped breaker, or the system turning on and off every few minutes.
On an AC repair visit the technician measures motor current, checks bearing play and blade clearance, tests the capacitor and contactor, and looks at the compressor mounts and the line set. You hear what was found and why a repair is recommended, and you get a written estimate before any work begins.
A local note on salt air and wind
In Daly City and on the ocean side of the territory, wind and salt air are hard on outdoor units. Salt corrodes cabinet screws, fan guards and mounting brackets, so panels work loose and start to rattle, and the same wind drives leaves and litter into the fan grille. In homes with the living space over a ground-floor garage, the furnace and its cooling coil usually sit in the garage, which is why a summer noise “in the garage” is so often the blower. A yearly AC maintenance visit tightens the hardware and catches a wearing motor before it stops.