Troubleshooting guide
The sound tells you most of the diagnosis. Match what you are hearing to the table below, then read the two sounds that mean stop using it today.
Work out first whether the sound happens at ignition, continuously while running, or at shutdown. That timing narrows it more than the sound itself does.
| What you hear | When | Usual cause |
|---|---|---|
| Single loud boom or bang | As it lights | Delayed ignition. Gas pools before igniting. Stop using it. |
| Continuous squeal or screech | Whenever the blower runs | Blower motor bearing, or a slipping belt on an older belt-driven unit |
| Metal scraping on metal | Whenever the blower runs | Blower wheel loose on its shaft or fouling the housing. Shut it off. |
| Rattling or buzzing | While running | A loose access panel, screw or section of ductwork vibrating |
| Light ticking or popping | Start and stop | Sheet metal ductwork expanding and contracting. Normal. |
| Rumbling that continues after heat stops | After shutdown | Burners staying lit when they should not, or a dirty pilot |
| Steady hum, no start | On a call for heat | Transformer or a failing run capacitor |
| Rapid clicking, no start | On a call for heat | Ignitor trying and failing to light, or a control board lockout |
If the furnace has locked itself out after several failed attempts, most units will retry after an hour or after the power is cycled. Repeated lockouts are a fault to report, not a switch to keep flipping.
Most furnace noises are an annoyance. These two are not.
A boom or bang as it lights. This is delayed ignition. Gas is entering the combustion chamber and collecting for a moment before something ignites it, so what should be a quiet light becomes a small explosion. It is usually caused by dirty burners, a weak ignitor or a gas pressure problem. Each event puts a shock through the heat exchanger, and a cracked heat exchanger is the fault that puts combustion gases into the air you breathe. Turn the furnace off at its switch and book a technician rather than living with it for the rest of the winter.
Metal scraping on metal. The blower wheel is either loose on the motor shaft or has moved far enough to catch the housing. Running it in that state turns a bearing or wheel replacement into a motor and housing replacement, and it can shear the shaft. Shut it off at the furnace switch and leave it off.
Neither of these is a wait and see. Both get worse with every cycle.
Two sounds get reported constantly and are almost always nothing.
Ticking and popping in the ducts. Sheet metal expands when hot air enters it and contracts as it cools, and it does that against the joists and straps holding it. You hear it a minute or two after the heat starts and again a few minutes after it stops. It is louder in a house with long duct runs under a floor, which is most of the older East Bay stock. Nothing is wrong.
A brief whoosh and a low roar on ignition. That is the burners lighting and the inducer drawing combustion air. A furnace is not silent. The question is whether the sound is a smooth roar that settles, which is normal, or a sharp single bang, which is not.
A useful test for anything you are unsure about: does the sound change when the blower runs on its own? Set the thermostat fan to ON with the heat off. Anything you still hear is the blower or the ducts. Anything that disappears is combustion.
Two local patterns change what a noise is likely to mean here.
The first cold week is when everything announces itself. A furnace around here can sit unused from April to October. The bearing that is about to fail was already failing in spring; October is simply when you switch it on and hear it. If you are reading this in early autumn, run the heat for fifteen minutes now, while a technician can still come out the same week rather than during the first cold snap when everyone calls at once.
A lot of older housing here has wall or floor furnaces. Much of Alameda, San Leandro and the older parts of Oakland still runs gravity wall units rather than central forced air. Those have no blower at all, so every noise in the table above that involves a blower simply does not apply. What you get instead is expansion ticking from the metal grate and, on units with a standing pilot, a soft continuous hiss that is normal. A gravity furnace that has started making a roaring or rumbling sound is worth an inspection, because it has very few parts that can make a new noise.
Replacement means permits. Alameda County jurisdictions require a permit for furnace replacement. It forces an inspection of the venting and gas connections, which is exactly what you want checked on a unit that has been booming on ignition.
FAQ
That is delayed ignition. Gas is collecting in the combustion chamber for a moment before it lights, so the ignition happens all at once instead of smoothly. Dirty burners, a weak ignitor or a gas pressure fault are the usual causes. It is worth acting on quickly, because the repeated shock is hard on the heat exchanger, and a cracked heat exchanger is a combustion gas problem rather than a comfort one.
Light ticking or popping that starts a minute or two after the heat comes on, and again a few minutes after it stops, is ductwork expanding and contracting with the temperature change. It is normal and needs nothing. What is not normal is a single sharp bang at the moment the burners light, which is a different sound and a different problem.
A continuous squeal or screech whenever the blower runs is usually a dry or failing blower motor bearing. On older belt-driven units it can be the belt slipping on the pulley instead. Neither is an emergency in the way delayed ignition is, but a bearing that is allowed to seize can take the motor with it, so it is cheaper to deal with early.
A steady hum with no ignition usually means the control transformer or a run capacitor is powered but something downstream is not completing. The furnace is trying. Before calling, check the obvious interlocks: the blower door fully closed, the furnace power switch on, and the thermostat calling for heat. If those are all correct, the fault is inside the cabinet and needs a technician.
It depends entirely on the noise. Duct ticking and a bit of rattling can wait indefinitely. A boom on ignition or metal scraping on metal should not, because both get worse every cycle and both can turn a modest repair into a replacement. If you are not sure which category you are in, the fan-only test in the section above separates blower noises from combustion noises in about a minute.
Local repair