Most air conditioner noises are a loose part or a worn part announcing itself early, and the fix is cheap if you act on it. A few of them mean the system is damaging itself while it runs, and the right move is to shut it off at the thermostat before you pick up the phone. This post sorts the common sounds by what they usually are and which category they fall into.
Buzzing
A buzz is almost always electrical, and it almost always comes from the outdoor unit.
- Buzzing with the fan not spinning. The contactor has pulled in and power is reaching the unit, but the fan motor or compressor cannot start. Usually a failed capacitor. The motor sits there humming and heating up. Shut the system off at the thermostat and call; running it like this cooks the motor.
- Buzzing that stops when the unit starts. A contactor with pitted contacts chatters for a moment before it seats. It will fail eventually, and it is an inexpensive part to replace during a service visit.
- Constant low buzz while running. Often the contactor coil or a fan motor with worn bearings. Not urgent; mention it at the next visit.
- Buzzing from inside the house. Usually the transformer or the relay board in the air handler. If you also smell anything hot or burnt, shut it off.
Rattling
Rattling is mechanical: something loose that vibrates when the unit runs.
- Rattling from the outdoor unit. Panel screws back out over years of vibration, and a loose panel rattles against the cabinet. Twigs and leaves fall in through the top grille and hit the fan blade. Both are easy. A rattle that is deeper and comes from the base is often the compressor mounts, which is a tech job.
- Rattling from the ducts. In SFV ranch homes with original attic ductwork, the metal trunk lines expand and contract and the joints work loose. You hear it at startup and shutdown as a pop or a rattle overhead. Harmless by itself, but loose joints leak air, and in a 130°F attic that leak costs you cooling.
- Rattling from the indoor unit. A blower wheel with a loose set screw, or debris in the blower housing. Let a tech look, because a loose wheel can hit the housing and take out the motor.
Hissing
Hissing gets people worried because it sounds like a leak, and sometimes it is.
- Brief hiss at startup or shutdown. The refrigerant pressures equalizing through the metering device. Normal on most systems, and on some it is quite loud for a few seconds.
- Steady hiss from the outdoor unit or the line set. Possible refrigerant leak. The system loses capacity as the charge drops, and running a compressor on low charge damages it. Turn the system off and call.
- Hissing from the indoor unit that is loud and constant. Same story, often at the coil or a brazed joint.
- Hissing at a register or from the ceiling. Air escaping a duct joint or a gap around the boot. This is a duct problem, not a refrigerant one.
- A loud screaming hiss from the compressor. This is a high-pressure sound and the unit should be shut off immediately. Do not restart it until a tech has looked at it.
Other sounds worth knowing
- Clicking at the thermostat or the air handler when the system starts and stops. Normal relay operation.
- Rapid clicking from the outdoor unit that never turns into a start. A control problem or a failing contactor.
- Squealing. On older units with a belt-drive blower, a worn belt. On direct-drive units, motor bearings. Either way, plan a visit.
- Grinding. Bearings that have failed. Shut it off. A motor running on failed bearings can seize and take the capacitor and sometimes the board with it.
- Banging or hard clunk at startup. A compressor that is slugging liquid refrigerant, or a mount that has broken. Shut it off and call.
- Bubbling or gurgling. Refrigerant moving through a low system, or water in the condensate line. Not a shutdown, but a call.
What you can check yourself
Stick to what is on the outside of the cabinets.
- Look at the outdoor unit while it runs. Is the fan spinning? Is anything sitting on the grille or lodged against the fan blade? Are the panel screws tight? Cut power at the disconnect before you reach in to pull out a twig.
- Check the pad. A condenser that has sunk on one side after years on a dirt side yard, common behind the older houses in Pasadena, CA and Altadena, will rattle and vibrate. Shimming it level is a tech job because the line set does not like being bent, but noticing it is not.
- Change the filter. A starved blower makes a whistling or roaring sound at the return.
- Tighten register grilles. A screwdriver fixes most rattles that come from a vent.
- Check the thermostat. If the sound coincides with the system starting every few minutes, the real problem may be short cycling.
Do not open the electrical compartment on the outdoor unit. Pulling the disconnect cuts the high voltage, not everything: the low-voltage control wiring is fed from the indoor unit and can still be live. The capacitor holds a charge with the power off too, and it is the part most likely to be involved in a buzzing complaint.
When to call a licensed tech
Shut the system off and call the same day for: grinding, a loud screaming hiss, buzzing with the fan not spinning, a hard bang at startup, or any electrical noise with a burnt smell. These are the sounds that get worse by the hour when the unit keeps running. Book a visit within the week for: a rattle from the outdoor unit you cannot find, a steady hiss anywhere, squealing, or a duct rattle that is new. Our AC repair visit starts with listening to the unit run, because most of these sounds are easier to identify in person than to describe over the phone.
A lot of these noises show up on the checklist of an annual HVAC maintenance visit before they become a complaint. A contactor that is pitted, a capacitor that is weak, a fan motor that is starting to drag, all of those get measured. And if the sound is in the attic, a duct repair that reseals the joints stops the rattle and the air loss at the same time.
Common questions
Is a buzzing AC dangerous?
Usually not to you, but often to the unit. A humming motor that cannot start draws high current and heats up fast. Turn it off at the thermostat and let a tech test the capacitor before it takes the motor with it.
Can I ignore a hissing sound if the house is still cool?
For a short startup hiss, yes, that is normal. For a hiss that continues while the unit runs, no. A slow leak keeps the house cool right up until it does not, and by then the compressor has been running on low charge for weeks.
Why did my AC get louder as it got older?
Vibration loosens fasteners, bearings wear, fan blades collect grime and go out of balance, and the compressor mounts soften. None of it happens overnight, so the change is easy to miss. If it sounds different this summer than last summer, that is worth a look.
If your AC is making a sound you do not recognize, book a $69 diagnostic. We tell you the fee before booking and credit it toward the work. Call the Boost Services team at 818-277-5056 or send us a message and we will usually be out within a few hours.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.