Buzzing, Banging, Squealing: What Your AC and Furnace Noises Mean

Quick answer: Buzzing or humming from the outdoor unit often means a failed capacitor or contactor. A bang when the furnace starts can be delayed ignition. Squealing usually points to worn motor bearings, and grinding means turn the system off. Hissing can signal a refrigerant or duct leak, and gurgling often means a drain issue. New or louder sounds are early warnings; a quick inspection can prevent a breakdown.

Your heating and cooling system makes a certain amount of normal noise: the hum of the fan, the click of a relay, the rush of air from the vents. When a new sound shows up, it's often the first warning that a part is wearing out. Catching it early can mean a simple repair instead of a breakdown on the hottest or coldest day of the year.

Quick noise guide

Buzzing or humming

The outdoor unit should have a steady hum and a spinning fan. If it hums but the fan doesn't spin, or you hear a loud electrical buzz, the culprit is often a failed capacitor (which helps motors start) or a pitted contactor. Both are common summer failures. Turn the system off. Continuing to run a compressor that can't start properly can damage it. Never poke the fan with a stick to "help" it start: there's high voltage inside.

Banging or booming

A loud bang when the furnace lights can be delayed ignition: gas builds up for a moment before igniting, often because burners are dirty or the igniter is weak. It stresses the heat exchanger over time and should be inspected promptly. Lighter popping sounds from ductwork as metal heats and cools are usually harmless.

Squealing or screeching

High-pitched squealing usually means worn motor bearings in the blower or outdoor fan motor, or a worn belt on older furnaces. It tends to get louder until the motor fails. Schedule a repair before it quits during a heat wave or cold snap.

Grinding or scraping

Metal-on-metal grinding often means failing bearings or a blower wheel rubbing its housing. Turn the system off and call. Running it can cause more damage.

Clicking

One click at start or stop is normal. Repeated clicking without the system starting can be a failing igniter, relay, control board or thermostat issue.

Hissing

Hissing from the outdoor unit or refrigerant lines can signal a refrigerant leak, often accompanied by poor cooling or ice on the lines. See why is my AC freezing up. Hissing at vents or in the attic may be leaky ductwork, which wastes energy.

Rattling

Loose access panels, screws, debris inside the outdoor unit (sticks, acorns) or a loose fan blade can rattle. With the power off at the disconnect, check for debris and loose panels. If the rattle persists, call for service.

Gurgling or bubbling

Gurgling near the indoor unit often relates to the condensate drain (a partial clog or poor slope). Bubbling in refrigerant lines can indicate low charge. See AC leaking water inside.

Prevent noisy breakdowns

Many noisy failures (weak capacitors, worn motors, dirty burners, loose parts) are caught during a tune-up. Our technicians test capacitors, check motor amperage, tighten electrical connections and inspect burners. Read what happens during an AC tune-up.

• $89 tune-up special for heating or AC
• Prime Comfort Club: $19/month for two tune-ups a year, priority scheduling and 10% off repairs

Normal sounds you don't need to worry about

Not every sound means trouble. These are usually normal:

• A single click when the thermostat calls for heating or cooling, and again when the cycle ends.
• The whoosh of the gas burners lighting on a furnace.
• A steady hum from the outdoor unit and the sound of the fan.
• Air rushing from vents (though very loud airflow can indicate duct restrictions).
• Ticking or popping in metal ducts as they expand and contract.
• On heat pumps, a whoosh and brief silence during defrost, sometimes with steam outside.

The key is change: a new sound, a louder version of an old sound, or a sound paired with poor heating or cooling. When in doubt, record a short video and send it to us on WhatsApp. Hearing the sound helps us bring the right parts the first time.

Hear something new? Let us listen

A short phone video of the noise sent on WhatsApp helps us diagnose it faster. We serve Brick, Howell, Eatontown, Willingboro and all of Ocean, Monmouth and Burlington counties with same day AC repair and heating repair. Message us or call for an upfront price before any work begins.