No heating system is completely silent, and most of the sounds a furnace or boiler makes are ordinary. A soft whoosh when the burner lights, a steady hum from the blower, and ticking or light popping from ducts and pipes as the metal heats up and expands are all normal. What deserves attention is a new sound, a sound that keeps getting louder, or one of the few noises that mean the system should be shut off until someone looks at it.
Those few are worth knowing before anything else:
- A loud boom when the burner lights. This is usually delayed ignition, gas building up for a moment before it catches. Turn the heat off at the thermostat and have the furnace checked before you use it again.
- Any unusual noise together with a gas smell or a carbon monoxide alarm. Get everyone out of the house without operating switches, and call the gas company or 911 from outside.
- Rumbling that continues after the burner should have shut off. This can mean the gas valve is not closing fully and fuel is still burning. Turn the system off at its service switch and call. If the flame keeps going with the power off, leave the house and call the gas company from outside.
Furnace noises
A boom or bang when the burner lights
A gas furnace should light with a quiet whoosh as the flame carries across the burners. A boom means gas collected in the burner area before it lit, so it lit all at once. The usual causes are dirty or rusted burners that keep the flame from carrying from one burner to the next, a weak or misplaced igniter, or a gas pressure problem. Each of those small explosions stresses the heat exchanger, and over time delayed ignition can crack it. This is not a noise to live with.
Not every bang comes from the burner. A single pop from the ductwork when the blower starts or stops is usually sheet metal flexing as the air pressure changes, sometimes called oil canning, and it is generally harmless. To tell the two apart, stand near the furnace as a cycle starts: a burner boom happens right at ignition, before the blower comes on, while a duct pop comes when the blower starts, usually a minute or so later.
Squealing or screeching
A high-pitched squeal usually comes from a motor. Most modern furnaces have a direct-drive blower, with the fan wheel mounted on the motor shaft, and a squeal there tends to mean worn motor bearings. Older furnaces often have a belt-driven blower, where a worn, glazed or loose belt can squeal every time it starts. The other usual source is the inducer motor, the small fan that pushes exhaust out at the start of each cycle. A squeal that starts before the burners light points to the inducer, and one that starts with the airflow at the registers points to the blower. Either way, a squealing bearing is usually on its way out, and it is better to replace that motor on your schedule than after it seizes on a cold night.
Rattling
Rattles are often simple: a loose access panel, a cabinet screw that has backed out, or a section of ductwork vibrating when the air starts moving. Check that the furnace doors are seated firmly, and listen along the ducts you can reach. A rattle from inside the furnace at startup can come from the inducer motor, where a loose mounting or a damaged fan wheel makes noise until it gets up to speed.
A cracked heat exchanger can also tick or rattle as the metal expands and the crack shifts, so a persistent rattle inside the cabinet should not be ignored. More often, though, a rattle is something loose, and only an inspection of the heat exchanger and a combustion test can confirm a crack. A crack can let combustion gases, including carbon monoxide, into the air the blower sends through the house, so a furnace with a confirmed crack is normally shut down. On an older furnace, that is usually the moment to weigh repair against replacement.
Clicking without the burner lighting
A click or two at the start of a cycle is normal: relays on the control board switching and the gas valve opening. Repeated clicking without the burners ever lighting is different. On furnaces with spark ignition, it is the igniter sparking over and over without lighting the gas. On hot surface ignition furnaces, it is the control board running through ignition attempts that are not succeeding, which typically ends in a lockout after a few tries. The cause could be the igniter, the flame sensor, the gas supply or a pressure switch; our guide to a furnace blowing cold air walks through each one and what you can safely check.
Humming or buzzing
A low hum is part of a running furnace, since the blower motor, the inducer and the transformer that powers the controls all hum a little. A hum that gets noticeably louder, or a buzz from a motor that is not spinning, points to an electrical problem. A failing capacitor is the classic case: the motor is getting power but cannot start, so it sits there humming and heating up. Turn the system off and call. Capacitors hold a charge even with the power off, which makes this strictly a technician's job.
Whistling
Whistling almost always means air is being forced through an opening that is too small. The usual culprits are a dirty or overly restrictive filter, a missing cover on the filter slot, closed or blocked supply registers, and return ducts too small for the furnace. Start with the filter, the registers and anything blocking a return grille. If the whistle continues after that, the ducts may be undersized, which also makes the furnace run hot and is worth having measured.
Boiler and radiator noises
Boilers make a different set of sounds because they move water or steam instead of air, so it helps to know which kind you have. A steam boiler has a glass tube on the side showing the water level, and its radiators often have a small air vent on one end. A hot water boiler has a pressure and temperature gauge and a circulator pump on the piping, and feeds either radiators or baseboard.
Kettling
Kettling is a rumbling, popping or hissing from the boiler itself, much like a kettle about to boil, and the comparison is accurate. Water is flashing to steam in spots on the heat exchanger that are running too hot, and the bubbles collapse as they move into cooler water. In a hot water boiler, the usual cause is scale or sediment coating the inside of the heat exchanger, which insulates the metal so the water touching it overheats. Low flow does the same thing: a weak circulator, a partly closed valve or air in the system can keep water from moving through the boiler fast enough.
Scale builds faster when fresh water keeps entering the system, usually through a leak the automatic fill valve keeps topping up. A technician can find the leak, fix the flow and, depending on the boiler, flush or clean it. On an old boiler with heavy scale, kettling tends to come back and the scale keeps wasting fuel, and at that point boiler replacement is worth a look.
Banging and clanking in steam systems
Steam heat is mostly found in older houses, and there are plenty of those here: Census estimates show that about half the housing in Everett and Newton was built before 1940. The banging that comes with steam is mostly water hammer. Fast-moving steam meets a pocket of condensed water in a pipe, slams it against a fitting, and the pipe bangs. Steam pipes are supposed to slope so condensate drains back to the boiler, and when a pipe sags, a hanger fails or the house settles, water collects in the low spot and the banging starts.
Radiators are the other common source. On a one-pipe system, steam going up and water coming back share the same pipe, so a radiator valve left partly open can trap water in the radiator; those valves should be fully open or fully closed. A one-pipe radiator should also tilt slightly back toward its valve so water can drain, and if it has settled the other way, a thin shim under the feet at the far end is a simple fix. Wet steam, meaning water carried up with the steam, also causes banging. Its usual sources are a water level set too high, dirty or oily boiler water, or near-boiler piping that does not match the manufacturer's requirements, all technician fixes.
Gurgling in hot water systems
Gurgling, or the sound of water trickling through the pipes, usually means air is trapped in a hot water system. It gets in after work on the system, through small leaks, or as dissolved air comes out of fresh water when it heats. Trapped air also blocks circulation, so a radiator may be warm at the bottom and cold at the top.
Bleeding radiators is a safe homeowner job on a hot water system. With the thermostat turned down and the radiators cool, hold a rag under the small bleed valve near the top, open it slowly with a radiator key or screwdriver, and close it as soon as water comes out instead of air. If the boiler's pressure gauge has then dropped well below where it usually sits, call for service rather than adding water yourself. Many baseboard systems have no bleed valves and are purged at the boiler by a technician. And this is for hot water only: never open anything on a steam radiator while the system is running. Gurgling that keeps coming back can mean a stuck automatic air vent near the boiler, or a leak letting air in.
Humming or grinding from the circulator pump
The circulator is the pump, usually mounted on the piping near the boiler, that moves hot water through the system. A faint, steady hum is normal. A loud hum while the radiators stay cold and the boiler gets hot can mean the pump has seized, and grinding or rattling usually points to worn bearings, a damaged impeller or air in the pump housing. A failing circulator can also cause kettling, because water stops moving through the boiler. Replacing one is a common repair, but it means isolating or draining part of the system, so leave it to a technician.
Ticking baseboard
Copper baseboard pipe expands as it heats and shrinks as it cools. Where it rubs against the hangers inside the cover, the edge of a hole through the floor or a tight bracket, it ticks or creaks for the first few minutes of a cycle and again as it cools. It is harmless. If it bothers you, the fix is giving the pipe room to move by adjusting the hangers or enlarging a tight hole.
What to leave to a technician
Beyond the filter, the registers, the furnace doors, bleeding hot water radiators and leveling a steam radiator, a noise is a technician's job. Burners, igniters, gas valves, motors, capacitors, control boards, the circulator and the boiler's water level and pressure settings are best left alone. A yearly tune-up is a good time to mention a sound you have noticed, and our HVAC maintenance checklist shows what that visit should cover.
If a sound is new, getting louder or on the shut-it-off list above, the next step is a heating service visit, where you get the estimate before any work starts.