Furnace Making Noise: What Each Sound Means and What to Do

Some furnace noise is normal: Trane and Lennox both describe a hum while it runs and a click as the ignition fires. A boom when the burners light, grinding metal, squealing, buzzing, or a new rattle is not, and the manufacturers send most of those to an HVAC technician. Your part is short: a clean filter, open registers, panels that are seated and latched, and a clear description of the sound.

Find your sound below, emergencies first. If the furnace won't start, see furnace won't turn on; if it starts and stops every few minutes, see short cycling; for a burning smell, see furnace smells like burning. Banging from radiators or heating pipes comes from a boiler: see banging pipes for steam-pipe hammer and how to bleed radiators for gurgling.

First, Check What Is Normal

Lennox counts a soft hum when the blower kicks on, a gentle click when the ignition fires up, and a brief whoosh of warm air through the vents as a system doing its job. Trane adds a click before ignition and a slight pinging or popping from the ductwork as it expands when the heat comes on, and says banging or clanging from the registers is just the ducts adjusting to temperature changes. American Standard puts crackling down to metal parts cooling after shutoff, and says a rattle while the system cools down is not usually cause for alarm.

What matters is a change. Trane says a furnace shouldn't make any loud noises, and that a normally quiet unit that suddenly gets loud should be shut off and serviced. American Standard suggests learning how yours normally sounds and noting any change in the type or volume of sound.

Most Likely Causes

1. A boom or bang when the burners light: delayed ignition. Carrier's and Bryant's owner's manuals say buildups on the main burners can cause faulty firing, and that this “delayed ignition” is “characterized by an alarmingly loud sound.” Goodman's service manual explains it: gas and air collect in the combustion chamber and light late. Both owner's manuals say to shut the furnace down and call your servicing dealer; Carrier's shutdown steps start with the thermostat at its lowest setting, then closing the external manual gas valve, then turning off the furnace's electrical supply. Trane says it needs immediate attention. Why a technician: the causes Goodman and Rheem list are gas pressure, burners or orifices out of position, a blocked cross-lighter, and a weak igniter, all in the burner compartment, and Goodman says furnace design makes delayed ignition extremely unlikely unless safety controls have been bypassed or tampered with. A startup bang can also be the heat exchanger: American Standard says cracks can cause banging and rattling when the furnace starts, and Trane lists banging or rattling among the warning signs (see cracked heat exchanger).
2. Scraping or grinding metal: the blower. Shut it off. Rheem's service tips for a noisy blower include a wheel rubbing its housing, loose blades, a wheel sitting wrong on the motor shaft, a broken motor bracket, and a bad motor. Carrier says grinding often means worn bearings or loose components, and that dealing with it early can prevent permanent damage. Goodman's instructions: for unusual or objectionable noises during heating, turn the heat off at the thermostat and contact a qualified service organization right away. Then turn off the service switch, which Trane says looks like a light switch on the furnace, a nearby wall, or a ceiling joist.
3. Buzzing, or a loud hum: electrical parts or a motor. Trane says buzzing is louder than the everyday hum and isn't normal; it points to a loose or failing transformer, a bad capacitor, or an old or poorly maintained blower motor. Lennox adds loose wiring, which can arc and is a fire hazard. If buzzing comes with a burning smell, flickering lights, or a tripped breaker, Lennox says turn the furnace off immediately and call a professional. A loud hum can be the blower or inducer motor (Trane), and a motor that hums but won't start often has a failed capacitor, which Carrier says not to touch. A buzzing or vibrating duct, Trane says, has loose connections; it isn't an emergency.
4. Squealing or screeching: blower bearings or a belt. Trane says a high-pitched squeal is likely the blower motor, whose ball bearings wear out; a technician may replace the bearings and lubricate the motor, or replace the motor. Lennox also points to bad bearings. American Standard names the motor or fan belt, Carrier lists a loose belt or failing motor bearing, and American Standard says a clunk often means a cracked belt. Trane notes a dirty or too-restrictive filter can also cause a high-pitched sound, so check it, then call.
5. Rattling: a loose panel or duct, or a broken part. Carrier says to look for loose connections in the ductwork or panels, and Lennox says to check that all access panels are closed and latched. With the furnace off at the thermostat and service switch, press each panel to seat it; Trane notes that closing an ajar blower door restores power to the furnace and gas valve. Goodman says loose or ill-fitting ducts can echo through the registers, and Daikin says leaky ducts can rattle and need a professional's inspection. Trane says a rattle with excessive vibration isn't normal and can be the blower wheel, the motor, or a loose bolt inside; Lennox says a rattle from the blower motor can mean something broke inside it. Not a loose panel? Call.
6. Repeated clicking, but the burners don't light: the igniter or controls. Bryant says clicking usually means the igniter or pilot is trying to light the burners and failing, often from a dirty flame sensor, a bad igniter, or a gas supply problem, and lists repeated clicking without starting among the most common signs of a failing igniter. Trane adds the control board, a loose connection, a safety switch, or the thermostat, and suggests noting whether the clicking comes at startup, at shutdown, or continuously. Check the thermostat, power, and gas supply as furnace won't turn on describes, and write down any red status-light pattern, as Trane advises. The igniter and flame sensor are the technician's; Trane says not to clean or replace the sensor yourself.
7. A rumble as the burners light. Trane traces a loud rumble at startup to dirty gas burners, to be cleaned at regular maintenance, and notes a furnace at full blast can sound like rumbling in the room above it. Rheem's tips list excessive gas pressure and incorrect venting for noisy burners. Trane doesn't recommend opening the furnace to clean burners yourself.
8. Whistling: restricted airflow. Trane says a whistle most likely points to airflow: change a dirty filter, open the registers, and keep furniture off the return vents. Carrier names a clogged filter or gaps in the ductwork, and Daikin says leaky ducts can whistle. Our filter guide shows the swap. Don't run the furnace with the filter out to test it; Carrier's owner's manual says that may damage the blower motor.

Heat Pump Sounds in Winter

A heat pump's outdoor coil runs 10 to 20°F colder than the outdoor air in winter, Trane explains, so it periodically defrosts. Near the unit you may hear a swooshing sound as the refrigerant reverses, the outdoor fan may stop, and the compressor's noise may rise or fall, with a cloud of steam; Trane calls all of it normal, along with a hum while the fan is stopped. Bryant puts a typical defrost at about 5 to 15 minutes; Trane's demand-defrost cycles usually take 3 to 5. Daikin describes the sound of water flowing.

Not normal: noise that doesn't settle within a few minutes, or loud noise outside defrost (Trane); grinding (Bryant); or screeching, banging, or gurgling, for which American Standard says turn the heat pump off completely and call. See heat pump blowing cold air for defrost and clearing snow safely.

What You Can Fix Yourself

• Replace a dirty filter with the size printed on the old one. ENERGY STAR says check it monthly and change it at least every three months; Carrier says many 1-inch filters need replacing every 30 to 90 days.
• Open every supply register and clear furniture and rugs off the returns.
• With the furnace off at the thermostat and service switch, make sure every access panel is seated and latched.
• Look over the ductwork you can see for a joint that has pulled apart, and show the technician.
• Write down the sound, when it happens, and any status-light pattern.

That's the homeowner's share. Carrier's owner's manual limits untrained owners to basic maintenance such as cleaning or replacing filters, and Bryant says never to remove access panels or test internal furnace components yourself.

When to Call a Pro

Call a licensed HVAC technician for every sound in the list above except a whistle that a clean filter and open registers fix, or a rattle that turns out to be a loose panel. Lennox says strange sounds such as clanking, banging, squealing, or prolonged hissing could signal a serious problem that needs immediate attention from an HVAC professional, and that it's best to turn the system off while you wait. American Standard says noting when the noise happens and what it sounds like helps the technician diagnose it faster.

Prevent It

Book a heating check-up every fall, as ENERGY STAR recommends. Its checklist covers the parts behind most of these sounds: tightening electrical connections, lubricating moving parts, cleaning and adjusting blower components, and checking gas pressure, burner combustion, and the heat exchanger. ENERGY STAR notes a dirty burner or cracked heat exchanger causes improper burner operation, and Trane says a regular tune-up can help eliminate furnace noises. The CPSC also calls for yearly professional inspection and service of the heating system, and CO alarms on every level of the home and outside sleeping areas.

Frequently Asked Questions

Why does my furnace make a loud bang when it turns on?

Usually delayed ignition, which Goodman describes as gas and air collecting in the combustion chamber before they light. Carrier and Bryant say to shut the furnace down and call your dealer. Heat exchanger cracks can also bang at startup, per American Standard, so don't run it until a technician has looked.

Why do my ducts pop when the heat comes on?

Trane calls a slight pinging or popping from ducts expanding as the heat comes on normal. A buzzing or vibrating duct is different: Trane traces that to loose connections.

Why is my furnace squealing?

Trane and Lennox point to worn blower motor bearings, and Carrier notes that older belt-driven units can have a worn or loose belt. Check the filter first, since Trane says a dirty one can cause a high-pitched sound, then call a technician.

Is it safe to keep running a noisy furnace?

A hum, a click before the burners light, and some duct popping are normal. For unusual or objectionable noises, Goodman says to turn the heat off at the thermostat and call for service right away, and Trane says the same for a furnace that suddenly gets loud.

The Bottom Line

Hums, clicks before ignition, and popping ducts are normal. A boom when the burners light means shut the furnace down and call; grinding means shut it off; buzzing with a burning smell, flickering lights, or a tripped breaker means turn it off now. Squeals, unexplained rattles, and clicking with no flame go to the technician. A whistle usually starts with the filter and registers, which are yours.

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