What That Noise from Your AC Probably Means

Your air conditioner makes plenty of normal sounds: the hum of the compressor, the whir of the fan, the click of the thermostat. But when something changes, that new noise is usually trying to tell you something. Different sounds point to different problems, and knowing what you're hearing can help you decide how quickly you need to act.

Here's a quick reference to the most common AC noises we hear from homeowners in Austin, Round Rock, and Cedar Park, what typically causes them, how serious they are, and what to do next.

1. Grinding or Metal-on-Metal

What it sounds like: A low, harsh grinding or scraping, like metal rubbing against metal.

What causes it: Worn bearings in the blower motor (indoor) or fan motor (outdoor). When bearings fail, the motor shaft grinds against its housing instead of spinning smoothly.

Severity: Urgent. Continued operation will destroy the motor and can lead to costly replacement.

What to do: Turn the system off and schedule AC repair as soon as possible. Running it will make the damage worse.

2. Squealing or Screeching

What it sounds like: A high-pitched squeal or screech, especially when the system starts or runs.

What causes it: On older belt-driven systems, a slipping or worn belt. On newer direct-drive units, a failing motor bearing. The sound often gets louder as the motor warms up.

Severity: Moderate. A belt can be replaced quickly. A failing bearing will eventually seize and damage the motor.

What to do: Schedule a service call. If you have a belt-driven system, a belt replacement is straightforward. If it's a bearing, catching it early can save the motor.

3. Banging or Clanking

What it sounds like: Loud metallic bangs or clanks, often rhythmic or occurring when the system cycles.

What causes it: Loose or broken parts: a loose fan blade, a broken compressor mount, or internal compressor damage. Debris inside the outdoor unit can also cause banging if it hits the fan.

Severity: Urgent. A loose or broken fan blade can damage the unit or become a safety hazard. Compressor banging can indicate serious internal failure.

What to do: Turn the system off immediately. Do not run it until a technician has inspected it. Call for AC repair right away.

4. Hissing

What it sounds like: A steady hiss or bubbling, often near the indoor or outdoor unit.

What causes it: Refrigerant escaping through a leak, or a faulty expansion valve. Refrigerant under pressure makes a hissing or bubbling sound when it leaks.

Severity: Urgent. Low refrigerant makes the system work harder, can freeze the evaporator coil, and can eventually cause compressor failure. Refrigerant leaks also pose environmental and safety concerns.

What to do: Turn the system off and call for service. Refrigerant leaks require a licensed technician. Do not attempt to add refrigerant yourself.

5. Buzzing

What it sounds like: A constant or intermittent electrical buzzing or humming, louder than the normal compressor hum.

What causes it: Loose electrical connections, a failing contactor, a bad capacitor, or loose panels or screws that vibrate. Sometimes it's debris or a loose part rattling against the housing.

Severity: Moderate to urgent. Electrical issues can cause the system to fail or create a fire risk. Loose parts are less serious but should still be addressed.

What to do: If the buzzing is new or has gotten louder, schedule an inspection. A technician can tighten connections, replace failing components, and rule out electrical problems.

6. Clicking

What it sounds like: Repeated clicking, often when the system tries to start or when the thermostat cycles.

What causes it: A faulty relay, a stuck contactor, or a failing capacitor. The click is the relay or contactor attempting to engage but not holding. A single click when the system starts or stops is normal; repeated clicking is not.

Severity: Moderate. The system may not start reliably, or it may strain the compressor on repeated start attempts.

What to do: Schedule a service call. Relay and contactor issues are usually quick fixes. Ignoring them can lead to compressor damage from repeated hard starts.

7. Rattling

What it sounds like: Light metallic rattling or shaking, often from the outdoor unit or ductwork.

What causes it: Loose screws or panels on the outdoor unit, debris (sticks, leaves, acorns) inside the condenser, or loose ductwork. In older homes, duct joints can rattle when airflow changes.

Severity: Minor to moderate. Loose panels and debris are easy to fix. Persistent rattling can indicate a loose fan blade or mounting, which is more serious.

What to do: Check that the outdoor unit is clear of debris and vegetation. If the rattling continues, have a technician inspect it during your next maintenance visit. Do not remove the condenser cover yourself; the fan inside can cause injury.

Pro Tip: When you hear an unusual noise, note when it happens: at startup, during normal run, or at shutdown. That detail helps your technician narrow down the cause quickly. "It squeals when it first turns on" points to a different problem than "it squeals the whole time it runs."

8. Gurgling or Bubbling

What it sounds like: Watery gurgling or bubbling, often from the indoor unit or refrigerant lines.

What causes it: Refrigerant flowing through the lines (normal in some systems) or a refrigerant line issue. It can also indicate air in the refrigerant circuit or a partially restricted line. Sometimes it's condensate draining through the drain line, which is normal.

Severity: Minor to moderate. Normal refrigerant flow can produce soft gurgling. Loud or persistent gurgling, especially with reduced cooling, may indicate a refrigerant problem.

What to do: If cooling is normal and the sound is soft, it may be nothing to worry about. If cooling has dropped or the sound is loud and constant, schedule an inspection.

When to Act

Some noises are more urgent than others. Grinding, banging, and hissing should prompt an immediate call. Squealing, buzzing, and clicking warrant a scheduled visit within a few days. Rattling and mild gurgling can often wait for your next maintenance visit, unless cooling has dropped or the sound has changed.

CG Service Pros serves Austin, Round Rock, Cedar Park, and the surrounding area. Our NATE-certified technicians diagnose AC noises quickly, explain what they find in plain language, and get your system quiet and running properly again. We stock common parts on our trucks so most repairs can be completed in a single visit.

Get Your AC Checked

If you're hearing something that doesn't sound right, don't wait for it to get worse. Early diagnosis often means a smaller repair and a longer life for your system.

Schedule AC Repair Today

Have questions? Call us at (512) 766-5079 or visit our contact page to schedule service in Austin, Round Rock, Cedar Park, and the greater Central Texas area.