A working air conditioner makes a steady rush of air at the vents, a low hum from the outdoor unit and a click when it starts and stops. When a system in San Leandro starts grinding, banging, buzzing or rattling, the sound itself usually points to the cause: a loose panel or debris in the outdoor unit, a worn fan blade or motor bearing, an electrical part struggling to start a motor, or, less often, a failing compressor or a refrigerant leak.
Which noise are you hearing?
| Sound | Where it comes from | Usual cause | How urgent |
|---|---|---|---|
| Rattling or vibrating | Outdoor unit or indoor unit | Loose panel screws, twigs or leaves in the fan, loose mounting | Check it; schedule if it continues |
| Grinding or scraping | Outdoor fan or indoor blower | Worn motor bearings, fan blade touching the housing | Switch off and call |
| Squealing or screeching | Indoor blower or outdoor fan motor | Failing motor bearings; a slipping belt on older belt-drive blowers | Schedule soon |
| Banging or clanking | Outdoor unit or indoor blower | Loose or broken fan blade, loose blower wheel, failing compressor | Switch off and call |
| Loud buzz or hum, fan not turning | Outdoor unit | Failed capacitor, seized fan motor or contactor problem | Switch off and call |
| Rapid clicking, system never starts | Outdoor unit, thermostat or control board | Contactor, relay, capacitor or thermostat fault | Call |
| Hissing or bubbling | Refrigerant lines or indoor coil | Possible refrigerant leak | Call; a leak does not fix itself |
| Loud, high-pitched shriek | Outdoor unit | Very high pressure inside the compressor or a serious refrigerant fault | Switch off at once and call |
| Whistling at vents | Supply registers or return grille | Dirty filter, closed or blocked vents | Check the filter and vents |
| Popping or ticking in ducts | Attic or crawlspace ductwork | Metal ducts expanding and contracting, loose duct sections | Usually harmless; call if new and loud |
Sounds that are normal
- One click as the system starts and another as it stops.
- A steady hum from the outdoor unit and a whoosh of air at the vents.
- On a heat pump in winter, a loud whoosh and a cloud of steam from the outdoor unit during defrost.
- On a ductless mini-split, a soft crackle as the plastic housing warms or cools.
What you can safely check
- Turn the power off first. Set the thermostat to Off, then switch off the breaker or pull the outdoor disconnect before you touch the unit.
- Look for debris. Look through the top grille of the outdoor unit for sticks, leaves or clippings, and clear plants and stored items so there are about two feet of open space around it.
- Tighten loose outer panels. Snug any loose screws on the outside access panels. Do not open the electrical compartment: capacitors can hold a charge even with the power off.
- Check the filter and vents if the noise is a whistle. A clogged filter or closed registers make air squeeze through small openings.
- Note where and when. Outdoor unit, indoor unit or ducts? At start-up, all the time or at shut-off? Tell the technician.
Oiling a noisy motor is rarely the answer. Most modern fan motors have sealed bearings and no oil ports, and a squeal or grind means the bearings are already worn.
When to stop and call
Switch the system off and call if you hear grinding, banging or a shriek, if the outdoor unit hums while the fan stays still, if you notice a burning or electrical smell, or if the breaker trips. Running equipment with a seized fan or a struggling compressor is how a small electrical repair turns into a much larger one. If the system is also no longer cooling, see AC not cooling.
What a technician checks
- Runs the system to reproduce the sound and find its source: outdoor fan, compressor, indoor blower or ducts.
- With the power off, spins the fan by hand to feel for rough or loose bearings, checks the blade for cracks or bends, and checks that it is tight on the motor shaft.
- Tests the capacitor against its rating, inspects the contactor and measures the amp draw of the motors.
- On the compressor, checks the amp draw, the start components and the mounts. If there is hissing, a leak search comes before any refrigerant is added.
- Indoors, checks the blower wheel for dirt or imbalance, the motor mounts and loose panels, and, if the sound is in the ducts, looks for loose or disconnected runs that may need duct repair.
You hear what failed and whether it is safe to keep running, with parts and labor priced before any work starts. See AC repair for the visit and the AC repair cost guide for what common repairs usually cost.
Local notes
Outdoor units in San Leandro, San Lorenzo and Castro Valley often sit in narrow side yards against a fence or plantings, where leaves and clippings collect in the fan. Many systems sit idle through the cool, foggy months and then run for hours on warm late-summer and fall afternoons, which is when worn bearings and weak capacitors start making themselves heard. A spring AC tune-up catches most of these early. If the noisy system is older and has needed several repairs, see our repair-or-replace guide.