Strange noises from your HVAC: a translation guide

Equipment talks. The trick is knowing what it's saying. Here's a field guide to the most common HVAC noises and what they usually mean.

Banging or booming on startup

Two suspects:

• Loose ductwork expanding and snapping when the blower starts. Usually harmless but annoying. A tech can add stiffeners or change the static pressure to fix it.
• Delayed gas ignition. A small explosion when gas accumulates a beat too long before lighting. This is a real safety issue — it can crack a heat exchanger over time. Call us.

Squealing or screeching

Almost always the blower motor or its bearings. A worn motor bearing makes a high-pitched squeal that gets worse over months. A failed belt (rare on modern equipment, common on older furnaces) squeals on startup and then settles. Either way, replace before it seizes — a seized motor takes the rest of the system with it.

Buzzing or humming at the outdoor unit (no fan spinning)

Almost certainly a failed capacitor. The unit is trying to start the fan motor but doesn't have the kick to do it. Shut the system off at the disconnect immediately — running it like this damages the compressor. This is a same-day fix for us, often under $300.

Hissing

• Steady hiss at the outdoor unit: refrigerant leak. Service call.
• Hiss from registers when the blower runs: often just high static pressure from a clogged filter or undersized return. Sometimes a torn flex duct.
• Hiss from the gas valve: not normal. Don't ignore. Call your gas utility and us.

Clicking

A single click when the system starts and stops is the contactor — that's normal. Continuous clicking, or clicking without the system actually starting, points to a control board, a bad relay, or a failed igniter on a gas furnace.

Gurgling

Usually the condensate drain. Air pulling through water in a P-trap that has dried out, or a partially clogged line. Easy fix, but worth catching before it backs up entirely.