Furnace Making Strange Noises? Don't Ignore These Warning Signs

Every furnace makes some noise — the burner kicks on, the blower spins up, ducts expand and contract. But specific sounds map to specific failures, and a few of them are the kind you really don't want to ignore. Here's what each one usually means.

Loud "boom" on ignition

This is the one I take most seriously. A loud thump or boom when the burner lights almost always means delayed ignition — gas is accumulating in the burner before the spark catches it. Causes: dirty burners, a weak igniter, a misaligned flame sensor. Left alone long enough, delayed ignition can crack the heat exchanger, which is a carbon-monoxide risk. Don't ignore this one. Have it checked the next day.

Squealing during operation

Almost always the blower motor — either a worn bearing, a slipping belt (on older belt-drive units), or a failing capacitor starting to wind down. Catch it early and you replace one part. Wait until the squeal becomes a grind and you replace the whole motor.

Rattling that comes and goes

Loose ductwork, a worn blower wheel set screw, or — on rare occasions — a panel that wasn't tightened down on the last service. Rattling alone isn't dangerous, but it's worth fixing because the underlying movement wears other components.

Clicking that doesn't stop

If the clicking happens before the burner lights, it's the igniter trying and failing to start. Could be a bad igniter, no gas, or a bad gas valve. If the clicking happens randomly during a cycle, it's usually the relays on the control board.

Whistling air sounds

Almost always a return-air problem — too small a return, blocked register, or a clogged filter that's making the blower work harder than designed. Easy fix, but worth fixing because it shortens blower life.

Banging metal sounds

Expansion and contraction of cold-rolled steel ductwork on every cycle. Annoying, not dangerous. A bit of insulation between the duct and the support straps usually quiets it down.

The two warnings to take seriously

• Boom on ignition. Heat-exchanger risk.
• Smell of gas with any noise. Leave the house and call your gas utility from outside, then call us once they've cleared the line.

We do furnace repair across the Bay Area on central, wall, single-stage, two-stage, and modulating systems, plus heat pumps. Same-day service is standard during cold snaps.

Hearing something that doesn't sound right? Call (650) 691-3065 and we'll come listen with you.