Eight noises, and what each one usually is

Delayed ignition. Gas is entering the burner and not lighting straight away, so a small amount builds up and then catches all at once. The usual causes are dirty or misaligned burners, a weak igniter, a partly blocked flue, or gas pressure that is not right.

It is the noise most often described to us as “it has always done that”, and it is the one least worth living with. It tends to get worse rather than better, and the force of it can damage the heat exchanger — which turns an ordinary repair into a replacement.

Turn the heating off at the thermostat and at the service switch beside the appliance, and ring somebody. This is not usually an emergency — it is a same-week job, not a middle-of-the-night one — but it should not run in the meantime.

A blower motor bearing that has run dry, or a fan wheel that has shifted and is touching its housing. On an older unit with a belt drive it can also be a pulley that has worked loose on the shaft.

A dry bearing is a part. A seized motor that has been run to destruction is a bigger part, and it can take the control board with it. The difference between those two outcomes is usually a few hours of running.

Switch it off and ring. If it is winter and the house is getting cold, say so on the phone — there is sometimes a way to run it briefly and safely while you wait, and that is a decision for whoever is coming rather than a rule we can print.

On a belt-driven blower, the belt — glazed, loose, or misaligned with its pulleys. On a direct-drive motor, a bearing at an earlier stage than the grinding above. On an outdoor unit in summer, occasionally the compressor under high head pressure, which is a different and more serious conversation.

Note when it happens. Only at start-up usually means a belt or a bearing under load. All the way through the cycle points elsewhere. That one detail on the phone changes what comes on the van.

It can run for now. Book it rather than ignoring it — a belt that goes on a January night means no airflow, and no airflow on a furnace means it heats, trips and locks out.

The appliance trying to light and failing. On most modern furnaces it tries three times and then locks out. A weak or cracked igniter, a dirty flame sensor, a pressure switch that is not closing, or a blocked condensate line on a high-efficiency unit are all common and all inexpensive.

The flame sensor is a thin metal rod that gets a film on it over a season. Cleaning it is a small job and it is behind a large share of these calls. It is not something to attempt yourself — it sits in the burner compartment beside live gas — but it is worth knowing the repair is usually small.

Resist resetting it over and over. Each cycle sends gas to a burner that is not lighting, and repeated attempts are the one way to turn a cheap fault into a real one. Switch it off and book it.

In summer, on an outdoor unit, this is very often a failed run capacitor — the compressor is trying to start and cannot. It is one of the cheapest parts on the system and one of the most common failures. On an indoor unit it can be a contactor, a transformer or a seized motor.

A motor drawing current and not turning gets hot. Left humming for a long stretch it can go from a capacitor job to a compressor job, and those are not remotely the same price.

Switch the cooling off at the thermostat and ring. Do not open the outdoor unit — a capacitor holds a charge after the power is off, and that is the whole reason this is not a home repair.

A panel that is not seated, a screw missing from a duct joint, a hanger that has worked loose, or sheet metal flexing as the system pressurises. If pressing on the duct or the cabinet changes the noise, that is what it is.

If the rattle is inside the blower compartment and does not change when you press on anything, that is more likely to be debris in the fan wheel or a balance weight that has come off — and that belongs with the grinding answer rather than this one.

Nothing urgent. Mention it at the next service and it gets sorted in ten minutes. It is worth saying out loud that this one does not need a call-out on its own, because it is the noise people most often pay to have looked at in isolation.

Air moving too fast through too small an opening. A blocked filter, a return that is undersized for the system, closed or blocked registers, or a duct that was always too tight for the airflow it carries.

It is the audible symptom of high static pressure, and high static pressure shortens the life of the blower, worsens every comfort complaint in the house, and stops any replacement equipment from performing. It is the noise most likely to be connected to a problem you have already accepted as normal.

Check the filter first — that is a genuine home fix and it is free. If it persists with a clean filter, it is worth having the static pressure measured rather than living with it.

Gurgling is usually air in the system, and bleeding it is a small job. Banging as the system heats up is often pipes expanding against a joist or a clip — annoying, harmless, and fixable. Persistent air that comes back within weeks is a different matter: something is drawing it in, and that is worth finding.

Hissing from a radiator vent while the system is filling is normal and expected — that is the vent doing its job. Hammering is not. It usually means water sitting where steam needs to travel, from a radiator or a pipe that has lost its pitch, and it is worth correcting because it is hard on the system.

Tell us which system you have and when the noise happens — filling, running, or shutting down. That single answer decides which of the above it is, and it is the one question we always have to ask.