What Each AC Noise Means
Your AC shouldn't make loud or unusual noises during normal operation. When it does, the type of sound is a diagnostic clue that tells our technicians exactly where to look. Here's a complete guide to AC noises and what they indicate.
7 AC Noises and Their Causes
- Banging or clanking — A loose or broken internal component, often a connecting rod, piston pin, or crankshaft inside the compressor. A banging noise from the outdoor unit usually means the compressor is failing. Urgency: HIGH — turn off system and call immediately.
- Squealing or screeching — A failing fan motor bearing or a worn belt (in older belt-driven systems). This high-pitched noise typically gets worse over time. If caught early, a bearing replacement or lubrication is a simple fix. Urgency: MODERATE — schedule service within 1–2 days.
- Buzzing — An electrical issue such as a failing capacitor, loose wiring, a malfunctioning contactor, or a failing compressor motor. Buzzing can also indicate a refrigerant leak at the service valve. Urgency: MODERATE to HIGH — have it diagnosed promptly.
- Clicking at startup or shutdown — Some clicking is normal during startup and shutdown. However, persistent or repeated clicking — especially without the system starting — indicates a failing relay, contactor, or thermostat signal issue. Urgency: MODERATE — schedule a diagnostic.
- Hissing or bubbling — A refrigerant leak. Hissing indicates gas escaping from a hole in the refrigerant lines; bubbling indicates a leak in the liquid line. This is never normal and always requires professional repair. Urgency: HIGH — call for service.
- Rattling or vibrating — Loose screws, bolts, or panels on the outdoor unit. It can also indicate debris (sticks, leaves, acorns) caught inside the unit. While often harmless, rattling can indicate a loose fan blade that could cause expensive damage. Urgency: LOW to MODERATE.
- Humming without starting — The compressor or fan motor is receiving power but can't start. This typically indicates a failed capacitor or a seized motor. The system draws power but can't run, which can damage electrical components. Urgency: HIGH — turn off and call.
Which AC Noises Are Emergencies?
Turn off your AC immediately and call for emergency service if you hear: banging or clanking from the compressor, hissing (refrigerant leak), or any noise accompanied by a burning smell. These indicate conditions that can cause further damage or safety hazards if the system continues running.
Our Noise Diagnostic Process
- Listen and locate — we identify the exact source of the noise (indoor unit, outdoor unit, or ductwork)
- Visual inspection — check for loose components, debris, and visible wear on moving parts
- Electrical testing — measure capacitor strength, contactor condition, and motor amp draw
- Refrigerant check — test for leaks if hissing or bubbling is present
- Provide diagnosis and repair options — upfront pricing with no obligation