An air conditioner that suddenly starts making a loud noise is communicating. The type of sound tells you what is wrong more precisely than any other symptom. A buzzing sound is electrical. A rattling sound is something loose. A squealing sound is a bearing or a belt. A grinding sound is the compressor destroying itself from the inside. A banging sound is a part that has come free and is hitting something. A hissing sound is refrigerant escaping as a gas. Each sound corresponds to a specific component or failure mode. The diagnostic process is simply listening, identifying the sound, and matching it to the cause. The repair follows directly from the diagnosis.
Some AC noises are harmless and can be ignored. A soft hum from the compressor is normal. A whoosh of air from the condenser fan is normal. A click when the system turns on or off is the contactor engaging and disengaging, which is normal. The noises that are not normal are the loud ones, the ones that were not there last week, the ones that make you stop what you are doing and walk over to the window to look at the outdoor unit. Those are the noises that mean something needs attention. Ignoring them leads to more expensive repairs. A bearing that is squealing today will seize tomorrow. A part that is rattling today will break off and damage something else tomorrow.
EPA WaterSense encourages homeowners to maintain their appliances to prevent failures. An unusual noise from your AC is a warning that maintenance is overdue. Addressing the noise promptly prevents the small repair from becoming a large one.
Buzzing or Humming: Electrical Problems
A loud buzzing sound from the outdoor unit, especially a buzz that lasts a few seconds and then stops, is the compressor trying to start and failing. The buzz is the compressor motor drawing locked-rotor current, the maximum current it can draw, because the rotor is not turning. The compressor’s internal thermal overload protector trips after a few seconds to prevent the motor from burning out. The buzz stops. After the compressor cools down, it tries again, and the cycle repeats. A buzzing compressor that does not start is almost always caused by a failed start capacitor. The capacitor provides the extra electrical boost to get the compressor turning. When it fails, the compressor hums but cannot start.
A failed capacitor should be replaced by a technician. The capacitor stores electricity and can deliver a dangerous shock even when the power is off. The replacement part costs $20 to $50. The labor costs $150 to $300. Continuing to let the compressor buzz against a failed capacitor will eventually damage the compressor windings, turning a capacitor replacement into a compressor replacement, which costs $1,500 to $3,500.
A buzzing sound from the indoor unit, the air handler, may be the blower motor or a transformer. A blower motor that hums but does not turn has a failed capacitor the same as a compressor. A transformer that buzzes loudly may be failing. Transformer buzz is a constant hum, not a cycling buzz. A failing transformer should be replaced by a technician.
Rattling: Loose Panels, Debris, or Failing Parts
A rattling sound is something vibrating against something else. The most common cause is a loose access panel, a loose screw, or a piece of debris that has fallen into the outdoor unit. Check the outdoor unit for loose panels. Tap each panel with your hand. If a panel rattles, tighten the screws that hold it. Check for sticks, leaves, or debris that may have blown into the unit and are rattling against the fan. Turn off the power at the disconnect switch before reaching into the unit.
A rattling sound that is rhythmic, changing with the speed of the compressor, may be a failing compressor mount. The compressor is mounted on rubber isolation feet inside the outdoor unit. If a mount has deteriorated, the compressor vibrates against the metal base. The rattling is most noticeable at startup and shutdown when the compressor torque changes. A failing compressor mount is not an emergency, but it should be replaced before the vibration damages the refrigerant lines connected to the compressor.
A rattling sound from the indoor unit is often the blower wheel. Over time, the blower wheel can become unbalanced from dust buildup or can work loose on the motor shaft. An unbalanced blower wheel rattles against the housing. Tightening the set screw on the blower wheel or cleaning the accumulated dust may stop the rattle. If the blower wheel is cracked or bent, it must be replaced.
Squealing or Screeching: Bad Bearings or Belt
A high-pitched squealing or screeching sound from the outdoor unit is a failing condenser fan motor bearing. The fan motor has sealed bearings that wear out over time. The squeal starts intermittently, only when the unit first starts, and becomes continuous as the bearing degrades further. A squealing bearing will eventually seize, stopping the fan. Without the fan, the condenser cannot reject heat, and the compressor will overheat and shut down. A squealing fan motor should be replaced. The motor costs $100 to $250. The labor costs $200 to $400.
A squealing sound from the indoor unit on an older furnace with a belt-driven blower is the blower belt slipping. The belt connects the blower motor to the blower wheel. Over time, the belt stretches and glazes, losing its grip on the pulleys. A slipping belt squeals on startup and may produce a burning rubber smell. Tighten the belt tensioner or replace the belt. A replacement belt costs $10 to $20. Most modern furnaces have direct-drive blowers with no belt. If your furnace was manufactured in the last 20 years, it likely does not have a belt.
Grinding: Compressor or Fan Motor Failure
A grinding or growling sound from the outdoor unit is the most serious noise an AC can make. It typically indicates a failing compressor. Inside the compressor, metal parts are grinding against each other. The compressor is destroying itself. There is no repair for an internally failing compressor. The compressor must be replaced, or the entire condensing unit must be replaced. Continuing to run the AC with a grinding compressor spreads metal debris through the refrigerant lines, contaminating the entire system. The repair cost escalates from replacing the outdoor unit to replacing the outdoor unit, the indoor coil, and flushing the refrigerant lines.
A grinding sound from the indoor unit is the blower motor bearings failing. The sound is similar to a squeal but lower in pitch and more metallic. A grinding blower motor should be replaced. The motor costs $150 to $400. If the furnace is more than 15 years old, replacing the entire furnace may be more cost-effective than replacing the blower motor.
Hissing or Bubbling: Refrigerant Leak
A hissing or bubbling sound from the indoor unit, the outdoor unit, or the refrigerant lines is refrigerant escaping from a leak. The hiss is gas escaping under pressure. The bubbling is liquid refrigerant boiling off as it escapes. A refrigerant leak is not a noise that can be ignored. The system is losing its ability to cool. The leak must be located and repaired by a technician. The repair involves recovering the remaining refrigerant, repairing the leak, pressure-testing the repair, evacuating the system, and recharging it. The cost is $300 to $1,500 depending on the location of the leak.
Frequently Asked Questions
Why does my AC make a loud bang when it starts?
A loud bang or thump at startup is a loose part that shifts when the compressor torque is applied. The most common cause is a loose compressor mount or a refrigerant line that is not properly secured and slaps against the cabinet when the compressor starts. The bang is not immediately dangerous, but the vibration can eventually wear a hole in the refrigerant line or damage the compressor connections. Have a technician inspect the compressor mounts and secure the refrigerant lines.
Why is my AC making noise after a power outage?
A power outage can cause a voltage surge when the power is restored. The surge can damage the compressor, the fan motor, or the capacitor. A compressor that was running when the power went out may have been damaged by the sudden loss and restoration of power. The noise after a power outage may be a damaged component. Turn the AC off at the thermostat and let it sit for 30 minutes. Turn it back on. If the noise persists, call a technician.
Should I turn off the AC if it is making noise?
Yes, if the noise is loud, grinding, banging, or hissing. Turn the AC off at the thermostat to prevent further damage. A grinding compressor is destroying itself with every revolution. A refrigerant leak is losing charge with every second. A buzzing compressor that will not start is overheating its windings. Turning it off stops the damage. Call a technician. The cost of the service call is less than the cost of the damage that continued operation will cause.
The Bottom Line
An AC making a loud noise is communicating a specific problem. A buzz that cycles on and off is a failed capacitor. A rattle is a loose panel or debris. A squeal is a bad bearing or belt. A grind is a failing compressor or motor. A hiss is a refrigerant leak. Identify the sound. Match it to the cause. Turn the unit off if the noise indicates a serious problem. Call a technician for electrical, refrigerant, or compressor issues. Tighten a loose panel or clear debris yourself. The AC that made a strange noise for a week and then stopped completely was not a mystery. It was a warning that was ignored.
Last modified: July 7, 2026
