Troubleshooting
AC Making Noise? What Buzzing, Hissing and Banging Sounds Mean
Every AC hums, whooshes and clicks a little. New noises are worth chasing: buzzing usually points to an electrical part, hissing to a refrigerant or duct leak, banging to something loose or broken, and squealing or grinding to a motor bearing or belt. Find where the sound comes from and when it happens, then use the table below.
Normal sounds versus problem sounds
A working central air system makes a predictable set of noises: a single click from the thermostat relay and the outdoor contactor at start and stop, a steady hum from the compressor, the whoosh of the outdoor fan, and airflow at the registers. Heat pumps also make a loud whoosh and a short hiss when they switch into or out of defrost in winter. A brief hiss or gurgle from the indoor coil for a minute after shutdown, as pressures equalise, is also normal on many systems.
What deserves attention is a sound that is new, louder than it used to be, or tied to a change in performance, such as weaker cooling or a tripping breaker. Before you dig into causes, note two things: where the sound is coming from (outdoor unit, indoor air handler, or the ducts) and when it happens (at start-up, while running, or at shutdown). Those two answers narrow the list more than anything else.
Sound-to-cause table
| Sound | Where and when | Most likely causes | Urgency |
|---|---|---|---|
| Buzzing | Outdoor unit, at start or while trying to start | Failed run capacitor, sticking contactor, compressor or fan motor that cannot start, loose wiring | Shut off soon; can overheat the compressor |
| Buzzing | Outdoor unit, unit off but buzzing | Contactor stuck partly closed or low-voltage chatter from a faulty thermostat signal | Turn off power and call for service |
| Clicking | Once at start and stop | Normal relay and contactor operation | None |
| Clicking | Rapid or repeated, unit not starting | Failing contactor or relay, weak transformer, thermostat cycling, a fan blade tapping debris | Moderate |
| Hissing | Near the coil or refrigerant lines, continuous | Refrigerant leak, especially if cooling is weakening | Call a technician |
| Hissing or whistling | At registers or ducts | Duct leak, undersized return, closed registers, clogged filter | Low to moderate |
| Screaming | Outdoor unit, high-pitched | Very high internal pressure or a failing compressor | Shut off immediately |
| Banging or clanking | Outdoor unit at start or running | Loose or broken compressor mount, broken internal compressor part, liquid refrigerant slugging | Shut off and call |
| Banging or thumping | Indoor blower | Loose or unbalanced blower wheel, broken motor mount | Shut off and call |
| Popping | Ducts at start and stop | Sheet-metal ducts flexing as pressure and temperature change | Usually harmless |
| Squealing | Indoor unit | Worn blower belt on older belt-drive units, dry motor bearings | Moderate |
| Squealing | Outdoor unit | Fan motor bearings failing | Moderate; replace before it seizes |
| Grinding or scraping | Indoor or outdoor motor | Bearings worn through, fan blade hitting the shroud | Shut off; continuing can destroy the motor |
| Rattling | Either unit | Loose access panel screws, twigs in the fan, loose ductwork | Low |
| Gurgling | Indoor unit while running | Condensate drain partly blocked or missing a trap | Low; see the drain guide |
What the main noises are telling you
Buzzing
The most common cause is a run capacitor that has lost capacity. The compressor or fan receives power but cannot get turning, so it hums or buzzes, overheats, and shuts down on its internal overload. If the outdoor fan is not spinning while the unit buzzes, treat it as urgent; running in that state can damage the compressor and is a frequent prelude to a tripping breaker.
Hissing
A steady hiss near the indoor coil or the copper lines, especially with weakening cooling or ice on the suction line, suggests a refrigerant leak. Air leaks in ducts and too little return area produce a softer, airy hiss or whistle at registers. The return vent guide covers return sizing.
Banging and clanking
Metal-on-metal impacts mean something is loose or broken. On the outdoor unit that may be a compressor mount, or inside the compressor itself, which is serious. Indoors, a blower wheel that has slipped on its shaft will thump and vibrate.
Squealing and grinding
Both usually come from motors. A squeal is often the first warning of a dry bearing; grinding is a bearing that is already failing. Motor replacement now costs far less than replacing a motor that has seized and damaged the fan or wheel.
Diagnostic sequence, safest checks first
- Locate and time the sound. Stand by the outdoor unit, then the indoor unit, then a few registers. Note whether it happens at start, while running or at shutdown. Record a short phone video; it is useful for a technician.
- Check the filter and registers. A clogged filter and closed supply registers cause whistling and can make the blower strain. Replace the filter and open registers.
- Tighten what you can see. With power off, tighten loose screws on access panels and the outdoor top grille. Loose sheet metal is a common rattle.
- Clear the outdoor unit. With power off at the disconnect, remove sticks, leaves and anything touching the fan. Look through the grille to see whether the blade is bent or loose on its shaft.
- Watch the outdoor fan. With power restored, the fan should start within a second or two of the compressor. If the unit buzzes and the fan stays still, switch the system off at the thermostat and call for service.
- Stop for anything urgent. Screaming, grinding, heavy banging, burning smells or a buzz with no fan movement all mean switching the system off and leaving it off until a technician has looked.
- Technician work. Capacitor and contactor testing, motor bearing and amp-draw checks, compressor diagnosis, leak detection and refrigerant work. Refrigerant handling requires EPA Section 608 certification.
The outdoor unit's service panel hides 240 V terminals and a capacitor that can hold a dangerous charge after power is removed. Leave it closed unless you are trained and equipped.
Typical repairs and how to keep it quiet
Typical US costs: capacitor $150–$400, contactor $150–$350, outdoor fan motor $350–$800, indoor blower motor $400–$1,200 depending on whether it is a standard or variable-speed (ECM) motor, and leak repair plus recharge $300–$1,500+ depending on the leak. Compressor faults are covered in the compressor replacement cost guide.
To keep noise down long term: change filters on schedule, keep the outdoor unit level and clear of debris, have motors and electrical parts checked annually, and do not let a system run for weeks with a new noise. If your system is simply loud by design, the sones to decibels converter helps when you compare quieter replacement models, which list sound ratings in decibels.
Frequently asked questions
Why does my AC make a loud click when it turns on?
One clear click at start and stop is the contactor or relay closing and opening, which is normal. Repeated clicking without the unit starting points to a failing contactor, relay or a low-voltage problem.
Is a hissing air conditioner dangerous?
A refrigerant leak is not usually an immediate danger in a ventilated space, but it will damage the compressor over time and should be repaired by a certified technician. A high-pitched scream from the outdoor unit is different: shut it off at once.
Why does my outdoor unit hum but the fan does not spin?
That is the classic sign of a failed run capacitor or a seized fan motor. Turn the system off to protect the compressor and have a technician test the capacitor.
Should my heat pump make a whooshing sound in winter?
Yes. During defrost the reversing valve shifts, making a whoosh, and you may see steam rising from the outdoor coil. That is normal operation.
Last reviewed 2026-10-28