Furnace making strange noises? What each sound means

Every furnace makes some noise. The question is which noises are the system doing its job and which are the system asking for help. After twenty years of listening to them, here's the short version.

A dull bang or boom a second after the furnace calls for heat is delayed ignition: gas builds up before it lights. It's a dirty burner or a weak igniter, and it's worth a visit, because that small explosion stresses the heat exchanger every time.

Squealing or screeching that rises and falls with the blower is a belt or a bearing. Older furnaces with belts can be adjusted. Newer direct-drive blowers with a squealing bearing are telling you the motor's on its way out.

A rattle that changes when you press on the cabinet is a loose panel or screw. Tighten it. A rattle from inside that doesn't change is a loose component or a crack somewhere, and that one needs a look.

Clicking at the thermostat and at the furnace when it starts and stops is normal. Clicking that repeats over and over without ignition is the igniter failing to light, usually a flame sensor that needs cleaning.

A high-pitched whistle when the blower runs is air being pulled through a gap, often around the filter door or a return duct that's undersized. Easy to find, easy to fix.

Rumbling after the burners shut off can mean gas is still burning in the chamber, which is a safety issue. If the rumble is new, turn the furnace off and call.

The takeaway: new sounds mean something changed. Most are small when caught early. If a noise started this week, don't wait for it to start next week too.