AC Making Strange Noises: What Each Sound Means

Buzzing: Electrical Issues ($150 to $500)

A persistent buzzing from the outdoor unit typically indicates an electrical problem. The most common causes are a failing contactor (the relay that controls power to the compressor), loose wiring connections, a failing capacitor, or a malfunctioning compressor that is trying to start but cannot. Buzzing that occurs only at startup and then stops is usually a capacitor issue at $150 to $400. Continuous buzzing while the unit runs points to a contactor or wiring problem at $150 to $500.

Buzzing from the indoor unit is less common but can indicate a problem with the blower motor, a loose transformer, or vibration from an improperly secured component. Indoor electrical repairs typically cost $150 to $400.

Urgency: Moderate. An electrical buzz is not an emergency, but it should be addressed within a week or two. A failing contactor or capacitor will eventually fail completely and leave you without cooling. Have a technician diagnose and repair it before it becomes a total failure, especially in the peak of summer when wait times for service calls increase.

Clicking: Capacitor or Contactor ($150 to $400)

Rhythmic clicking from the outdoor unit at startup is normal, as the contactor engages and the compressor starts. Rapid, repeated clicking where the unit tries to start but fails is a clear sign of a failing capacitor or contactor. The capacitor provides the initial electrical boost to start the compressor motor, and when it is too weak to do so, the contactor clicks on and off repeatedly as the system attempts to start.

Clicking from the indoor thermostat or air handler may indicate a relay issue or a problem with the control board. This is less common and can be harder to diagnose because the control board manages multiple functions. Control board repairs range from $200 to $600 depending on the specific component and the board availability.

Urgency: High. Repeated failed starts stress the compressor motor and can cause permanent damage. If your outdoor unit is clicking rapidly without starting, turn the system off at the thermostat and call for service. Running the system in this condition risks turning a $150 capacitor repair into a $2,000+ compressor replacement.

Hissing or Bubbling: Refrigerant Leak ($200 to $1,500)

A hissing sound near the indoor unit, the refrigerant lines, or the outdoor unit often indicates a refrigerant leak. The sound comes from pressurized refrigerant escaping through a crack or hole in the tubing, coil, or fitting. Bubbling or gurgling sounds can also indicate a leak, specifically where liquid refrigerant is passing through a constriction or a low-charge section of the system.

Not all hissing is a refrigerant leak. The expansion valve makes a normal hissing or whooshing sound as refrigerant passes through it during normal operation. This is a steady, consistent sound that occurs whenever the system is running. A leak-related hiss is often intermittent, louder, and may be accompanied by a chemical or oily smell near the leak point.

Urgency: High. A refrigerant leak gets worse over time, and low refrigerant causes the system to work harder, reduces cooling performance, and can cause the evaporator coil to freeze up. More importantly, running a system with critically low refrigerant can damage the compressor. Call for service within a day or two of noticing a new hissing sound. Repair costs depend on the leak location and severity. See our refrigerant recharge cost guide for pricing details.

Banging or Clanking: Loose or Broken Parts ($200 to $2,800)

A loud banging, clanking, or rattling from the outdoor unit usually means something has come loose inside the condenser cabinet. Common causes include a loose fan blade hitting the protective cage, a broken fan blade, loose mounting bolts on the compressor, or internal compressor damage where a connecting rod or piston has broken free. The repair cost depends entirely on what is loose or broken.

A loose fan blade or mounting hardware is a $100 to $300 repair. A broken fan blade requiring motor disassembly and blade replacement runs $200 to $500. Internal compressor damage (the most serious cause of banging) means the compressor is failing and will need replacement at $800 to $2,800. If the banging is accompanied by a loss of cooling, compressor failure is the likely cause.

Banging from the indoor air handler is less common but can indicate a blower wheel that has come loose on the motor shaft or an object (like a chunk of insulation) that has fallen into the blower housing. Indoor banging repairs typically cost $150 to $400.

Urgency: Immediate. Turn the system off immediately if you hear loud banging or clanking. Running the system with a loose internal component causes additional damage rapidly. A loose fan blade can destroy the condenser coil. A failing compressor with internal damage gets worse with every minute of operation.

Screeching or Squealing: Motor Bearings ($200 to $2,300)

High-pitched screeching, squealing, or grinding sounds almost always indicate failing motor bearings. This can occur in the outdoor condenser fan motor ($200 to $700 to replace) or the indoor blower motor ($500 to $2,300 to replace). The sound starts intermittently and gets louder and more frequent as the bearings deteriorate. See our fan motor cost guide for a complete breakdown by motor type.

A screeching that occurs only briefly at startup and then stops may indicate a belt issue on older belt-driven blower systems. Belt replacement costs $100 to $200 including labor. Most modern systems use direct-drive motors without belts, so this is mainly an issue for systems installed before 2000.

Urgency: Moderate to high. A motor with failing bearings will eventually seize completely, stopping airflow and potentially tripping breakers. Schedule service within a few days. The motor will continue to work for a while with bad bearings, but the longer it runs, the greater the risk of a complete failure during peak cooling demand.

Whistling: Airflow Restriction ($0 to $400)

A whistling or high-pitched whining from the indoor unit or ductwork usually indicates restricted airflow. The most common cause is a clogged air filter forcing air through a small remaining area at high velocity, creating a whistle. Other causes include closed or partially closed dampers, undersized ductwork, and gaps in duct connections where air escapes under pressure.

Check and replace the air filter first ($0 to $30). If the whistling continues with a new filter, have a technician inspect the ductwork for obstructions, disconnected sections, or design issues. Duct repairs run $100 to $400 for most residential issues.

Urgency: Low. Airflow restrictions reduce efficiency and comfort but do not cause immediate damage in most cases. Address within a few weeks to prevent the restricted airflow from causing the evaporator coil to freeze.

When to Turn Off Your AC Immediately

Not all AC noises require immediate system shutdown, but some do. Turn the system off immediately if you hear loud banging, clanking, or metal-on-metal sounds from the outdoor unit, as these indicate loose or broken internal components that cause additional damage with every second of operation. Turn it off if you hear a loud grinding sound that was not present before, which suggests a seized or seizing motor bearing that can overheat and damage surrounding components. Turn it off if the breaker trips when the system starts, which indicates a short circuit or ground fault that poses an electrical safety risk.

For all other noises, the system can usually continue running while you schedule service, though sooner is better. A buzzing or clicking sound can wait a few days for a regular service appointment. A hissing sound suggesting a refrigerant leak should be addressed within a day or two to prevent compressor damage from running on low charge. If you are unsure whether a noise is urgent, call an HVAC company and describe what you hear. Most can advise you over the phone on whether to shut down immediately or wait for a scheduled appointment.