Furnace Making Noise: Buzzing, Banging and Squealing

Work out whether it buzzes with the blower or without it

A buzz all the time, even when nothing is running. That is the transformer or a relay on the control board. A faint hum from a transformer is normal. A loud one that has got worse is a transformer on its way out.

A buzz only when the blower is running. The blower motor, or its capacitor on furnaces that have one. A motor that also starts slowly or struggles to get going is usually the capacitor, which is cheap but holds a charge, so that is a technician unless you know how to discharge one.

A buzz that changes when you press on the cabinet. That is sheet metal vibrating, and it is a common buzz. Tighten every panel screw and check the panel is seated properly rather than leaning.

A short buzz at the moment the gas comes on. Many gas valves hum as they open. That one is normal.

Decide which bang you have

A bang at the moment the burners light. Delayed ignition, as above. Rust or dust across the burner ports, a tired igniter or low gas pressure all let gas build up before it catches. The burners need cleaning properly and the ignition checking, and that means a technician. If the furnace is also slow to light, the igniter is a place to start: which igniter you have and how to test it.

A softer boom about half a minute later, when the blower starts. That is ductwork. A flat duct wall flexes as the pressure comes up, like the bottom of an oil can, and it pops. It is noise rather than danger, and it usually comes from a return that is too small or a filter that is choking it.

A boom when everything stops. The same duct, relaxing again.

Squeal with the blower, or a whistle through the vents

A squeal as the blower starts and settles down after. On an older belt drive furnace that is the belt, and belts are cheap. On a direct drive one it is usually a dry motor bearing, and a squeal that gets worse every week means the motor is going.

A steady high pitched whistle while air is moving. That is air being forced through something too small. Start with the filter, then the vents, then look at the gap around the filter itself, because air whistling past the edge of a filter that does not fit properly sounds exactly the same.

A whistle with weak airflow at the vents is the same fault that makes a furnace overheat, so it is worth following through: why a furnace short cycles.

Rattles are usually loose, clicks are usually normal

Rattling while the blower runs, silent when it stops. Panel screws first, then the ductwork where it leaves the furnace, then the blower wheel itself. Things do find their way into a blower.

Clicking at the start of a cycle. Relays clicking on and a spark igniter clicking are both normal. Continuous clicking with nothing lighting is a spark igniter trying and failing.

A knock or a tick from the pipes rather than the furnace. Hot ducts and pipes expand and rub where they pass through timber. Harmless, and the only cure is finding the spot.

A loud rumble as the burners light. That is worth a look, because Trane says it can mean dirty burners. It goes with the delayed ignition family rather than the fan one.