Furnace Noises: What That Sound Means and What to Do

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Most furnace noises fall into three groups: normal sounds you can live with, wear sounds that mean you should book a repair soon, and a short list that means you should turn the heat off now. This page helps you put the sound you are hearing in the right group, with a noise-by-noise table, the checks you can safely make yourself, and what a technician looks at. It is part of our Bay Area Winter Heating Help Center.

First, notice when the noise happens

A gas furnace runs the same sequence every time the thermostat calls for heat. The small draft inducer fan starts, the igniter or pilot lights the burners, the heat exchanger warms up for a short while, the main blower comes on to push warm air through the ducts, and then everything shuts down in roughly the reverse order. A sound tied to one step points to the parts working at that step. Stand near the furnace (in Bay Area homes it is usually in a hall closet, the garage or the attic) and note three things:

• Timing: right at startup, when the blower kicks on, the whole time it runs, or after it shuts off.
• Location: inside the furnace cabinet, in the ducts, or outside at a heat pump.
• Pattern: every cycle, only on cold mornings, or getting louder week by week.

A 20-second phone video with sound, taken with the furnace doors in place, is one of the most useful things you can hand a technician. Many Bay Area furnaces sit idle from spring until the first cold, damp night in late fall, so a motor bearing that dried out or a burner that rusted over the summer often makes its first noise on that night.

Noise-by-noise table

The sounds that mean turn it off

A boom when the burners light

This is usually delayed ignition. Gas flows to the burners for a moment before it catches, collects in the burner area and then lights all at once. Common causes are dirty or rusted burners, an igniter that is weak or out of position, a pilot flame that has shrunk, or low or unsteady gas pressure. Small puffs tend to grow and stress the heat exchanger and vent connections. The difference from duct noise: an ignition boom comes from the furnace cabinet a few seconds after the inducer fan starts and before the main blower comes on, while a duct pop comes from the ceilings or walls as the blower starts or stops. If you are not sure which it is, treat it as ignition. Our pilot light and ignition page explains the ignition systems involved.

Rumbling after shutdown

When the thermostat is satisfied, the burners should go out cleanly. A rumble that carries on afterward can mean gas is still reaching the burners or that combustion is dirty. Look for soot on the cabinet or around registers, and for a lazy, yellow flame through the sight glass where it should be crisp and blue. Confirm your carbon monoxide alarm works, turn the furnace off and call.

Grinding

Metal-on-metal grinding almost always means a motor bearing has failed or a blower wheel has come loose. A seized inducer motor stops the furnace from lighting at all. A seized or slow blower lets heat build up until the limit switch, the safety that shuts the burners down when the furnace overheats, trips (Carrier). Shutting it off early can save the parts a failing motor would damage.

Sounds that mean book a repair soon

Squealing

A failing bearing often squeals at startup and quiets as the motor warms, so people get used to it. It will not fix itself. Most modern furnace motors are sealed, so oil or spray lubricant does not help and can collect dust.

Clicking with no heat

A couple of clicks at the start of a cycle is normal, and furnaces with spark ignition tick steadily for a few seconds while they light. Clicking that repeats with no flame means the furnace is trying to light and failing, and after a few attempts most furnaces lock themselves out. Frequent causes are a cracked igniter, a dirty flame sensor (Lennox notes that a dirty sensor "will shut off the gas valve and the furnace") (Lennox) or a pressure switch that is not proving the vent is clear. If the furnace lights, runs briefly and shuts off over and over, read our furnace short cycling page, and if it will not start at all, our furnace won't turn on guide covers the checks.

Buzzing and rattling

A loud buzz with the blower not turning is usually a failing capacitor or a motor that cannot start; motors in that state run hot, so turn it off. For a rattle, check that the furnace door is seated: on most furnaces it presses a door safety switch, so a loose door can also stop the furnace running. A rattle from inside the burner or inducer area, or one that started after a boom, needs a technician.

Sounds that are usually normal

Metal ducts tick and pop as they warm up and cool down, especially in attics and crawlspaces that are cold on a winter morning. One loud bang from a duct as the blower starts is usually a flat duct panel flexing, often because the return is starved for air; it is not dangerous. A whistle usually means the system is short of airflow. ENERGY STAR advises checking the filter every month in heavy-use months and changing it at least every three months (ENERGY STAR). High-efficiency furnaces make water as they run, so a quiet trickle can be normal, but constant gurgling or sloshing is covered on our furnace leaking water page. And if the whoosh and steam are outside at a heat pump, that is usually defrost, explained on our heat pump winter problems page.

What you can safely do yourself

1. Replace the filter with the size printed on the old one, and open any closed registers. Move furniture and rugs off return grilles.
2. Make sure the furnace doors are fully seated and the outer screws are snug. Do not open the burner compartment or remove the inner panels.
3. Record a short video with sound, noting the time and whether it was a cold morning.
4. For anything in the "stop now" rows, set the thermostat to Off. For grinding or buzzing, also turn off the furnace service switch (it looks like a light switch on or near the unit).
5. Keep warm safely while you wait. Our space heater safety page covers the rules, and never use an oven or grill for heat.

How a technician tracks down a furnace noise

We run the furnace through complete cycles and watch each stage: inducer start, ignition timing, flame pattern, blower start and shutdown. We check burners, gas pressure, igniter and flame sensor, test motors and capacitors, and inspect the blower wheel, heat exchanger and vent. For airflow noises we measure static pressure and temperature rise to find the bottleneck. The CPSC recommends a professional inspection of all fuel-burning heating systems (CPSC), and many noises are caught early on a pre-winter tune-up.

A noisy furnace rarely means a new furnace. Motors, capacitors, igniters and burners are replaceable parts. Our furnace repair cost guide covers what those repairs typically involve, and our older guide to loud HVAC systems covers air conditioner noises too. Every repair we do carries a 1-year parts and labor warranty.

Book a furnace noise check

Frequently asked questions

Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.