A loud noise from your AC is the system telling you something is wrong before it actually fails. The good news: most AC noises trace to a small handful of causes, and the sound itself is a reliable diagnostic tool. The bad news: some of those noises mean you have hours, not days, before a $200 repair becomes a $2,000 one. Here is how to identify what you are hearing and what to do.
Banging — Stop Running It
What it sounds like: A loud single bang or repeated banging from the outdoor unit (or sometimes the indoor air handler).
What it usually is:
1. A loose or broken part inside the compressor. Most serious cause. The compressor is the largest component in your outdoor unit, and a broken connecting rod or piston can bang against the casing. This is unfixable as a compressor repair — full compressor replacement ($1,400–$2,500) or full system replacement is the next step.
2. A loose blower wheel or fan blade striking something. Less serious. The fan blade has come loose on its shaft or hit debris. A tech can usually rebalance and tighten in a single visit.
3. Loose mounting hardware. The unit itself is rattling against its pad. Cheapest fix.
What to do: Turn the AC off. Do not let a banging compressor keep running — each cycle does more damage. Call for diagnostic same-day.
Screeching or Squealing — Two Likely Causes
What it sounds like: A sustained high-pitched screech, especially when the system starts.
What it usually is:
1. Failing fan motor bearings. The blower motor (indoor) or condenser fan motor (outdoor) is running on dry or worn bearings. Sounds like fingernails on a chalkboard. Motor replacement is $400–$800.
2. Slipping belt (older units). Modern direct-drive systems do not have belts, but older systems still in service may. Cheap and quick fix.
3. High-pressure refrigerant in the compressor. Less common but more serious. The compressor's high-pressure switch is about to trip and shut down the system. Diagnostic needed.
What to do: Schedule a service call within a few days. Not as urgent as banging, but a screeching motor will fail completely in weeks rather than months.
Buzzing or Humming
What it sounds like: A loud electrical buzz or hum from the outdoor unit, sometimes with the unit failing to start.
What it usually is:
1. Failing capacitor. The single most common AC repair. The capacitor stores electrical charge and releases it to start the compressor. When it fails, the compressor tries to start, draws too much current, and you hear a hum but the compressor never spins up. $150–$300 to replace.
2. Loose electrical contactor. The relay that switches power to the compressor. Pitted contacts produce a buzzing sound. $150–$250.
3. Failing compressor. If the capacitor is replaced and the buzz returns within hours, the compressor is drawing excessive current. Bad sign.
What to do: Turn the AC off — running with a failing capacitor or contactor for hours can damage the compressor. Schedule same-day service.
Hissing — Refrigerant or Pressure Issue
Quiet hissing near the indoor coil or refrigerant lines: Refrigerant leak. Has cooling capacity dropped? Are vents blowing room-temperature air? If yes, this is your cause. Refrigerant repair is $400–$1,500.
Loud hissing from the outdoor unit: High-pressure release. The system is dangerously over-pressurized and the safety valve is venting. Turn the unit off immediately. Call same-day. Causes are usually a clogged condenser coil, a failing fan, or refrigerant overcharge.
Clicking — Normal vs Not
Single click when the AC starts and stops: Normal. That is the contactor engaging.
Continuous rapid clicking, no startup: The contactor or thermostat is rapidly cycling. Could be a thermostat issue, a low-voltage wiring problem, or a contactor failing. Diagnostic call. $89–$149 for the diagnostic, then repair cost depending on cause.
Clicking from the air handler with no airflow: Blower relay failure. The system is calling for air but the blower is not running. Service call.
Rattling or Vibrating
What it sounds like: A rattle, especially during startup, often from the outdoor unit.
What it usually is:
1. Loose panels or screws on the outdoor unit. Cheapest fix — tighten the screws.
2. Twigs, leaves, or debris inside the condenser cabinet. Common in fall. Power off and check.
3. Loose refrigerant lines vibrating against the cabinet. A tech can secure them with grommets.
4. Worn fan motor mounts. The motor is shifting in its mounts during startup. Mount replacement.
What to do: Turn off the unit, look for visible debris or loose panels. If nothing obvious, schedule a service call.
Whistling — Airflow Restriction
What it sounds like: A high-pitched whistle from vents or ductwork.
What it usually is: Restricted airflow. Filter is clogged, vents are closed, or there is a duct leak with air whistling through a small gap.
What to do: Replace the filter. Open all vents. If it persists, schedule a duct inspection — chronic whistling means high static pressure, which damages the blower motor over time.
What to Do Based on What You Hear
Banging: Turn off. Same-day call. Compressor at risk.
Screeching: Schedule within 3–7 days. Motor failing.
Buzzing/humming: Turn off if compressor is not starting. Same-day. Capacitor or contactor.
Quiet hissing + reduced cooling: Refrigerant leak. Schedule within a week.
Loud hissing: Turn off immediately. High pressure. Same-day.
Continuous clicking: Diagnostic call.
Rattling: Inspect for debris or loose panels first. Service call if needed.
Whistling: Filter and vents first. Duct inspection if persistent.
Get It Diagnosed
Same-day AC diagnostic across Central Texas — Pflugerville, Bastrop, Taylor, Elgin, Georgetown, Hutto, Manor, Cameron, Rockdale, Brenham, and our full service area. Call (737) 260-7255. Texas license TACLB00027491E. Flat-rate pricing on diagnosis and repair.