Boom. It comes from the hall closet a few seconds after the heat clicks on, loud enough that the dog looks up. Or maybe it is a thin squeal that starts with the fan and fades after a minute, or a rattle you only notice in the quiet of the night. A furnace making loud noises is giving you information, and each sound tends to point to a different part.

Some furnace sounds are normal. A soft whoosh when the burners light, a low hum from the blower and a few ticks as metal warms up are all part of a healthy cycle. The sounds below are the ones worth paying attention to, starting with the ones that mean you should switch the furnace off at the thermostat today.

Shut It Off Now: Three Sounds That Can't Wait

A boom or bang when the burners light. This is delayed ignition. Gas flows into the burner area but doesn't light right away, and when it finally catches, the small buildup of gas ignites all at once. Dirty burners, a weak ignitor or the wrong gas pressure are the usual causes. Each bang puts stress on the heat exchanger and burner assembly, and it is a sign combustion isn't happening the way it should. One unusual pop on the first run of the season might not repeat. A furnace banging noise every time it starts needs a technician.

Metal scraping or grinding. That is usually the blower wheel hitting its housing because it has worked loose, or a motor bearing that has failed. Left running, a loose wheel can destroy itself and the motor along with it.

A roar or rumble that keeps going after the burners shut off. A gas valve that isn't closing fully, or a flame that is burning badly, can be behind this, and either one needs a technician before the furnace runs again. It is also a sound worth pairing with a carbon monoxide check. If your CO alarm sounds at any point, get everyone outside and call 911, then follow our guide on what to do when a carbon monoxide detector goes off.

If you smell gas along with any sound, get out first, then phone Southwest Gas or 911 from the yard or the street.

High-Pitched Sounds: Squealing and Screeching

Furnace squealing almost always comes from something that spins.

On most modern furnaces, the blower motor drives the fan wheel directly. When its bearings start to wear, the motor can squeal, especially in the first minute or so after it starts. Older furnaces often use a belt between the motor and the blower, and a worn, loose or glazed belt squeals much like a car's fan belt.

The inducer motor, the small fan that pushes exhaust out before the burners light, can also squeal or whine as its bearings wear. Because it runs at the very start of each cycle, a squeal that comes before the main blower starts often points there.

In many Las Vegas homes, the same blower that heats the house in winter also moves all the cooled air in summer, so it has already put in a long season by the time the heat comes on. A new squeal in the fall is common for exactly that reason. None of these parts should be oiled or adjusted by a homeowner, because the work means opening the cabinet with power nearby. A squeal is rarely an emergency, but it rarely fixes itself either.

Rattling and Vibrating

A rattle is often the least serious sound on this list, and sometimes the easiest to trace.

Start with the outside of the furnace. An access panel that isn't seated properly, a loose screw or a filter door that buzzes against its frame can all rattle when the blower runs. Pressing gently on a panel while the furnace runs and hearing the noise stop is a good clue. Make sure the door panel is properly latched, since many furnaces won't run safely or at all with it ajar.

Ductwork can rattle too, particularly where a section has come loose from its hanger. If the sound is coming from a wall, ceiling or attic rather than the furnace itself, that is the more likely source.

A rattle that seems to come from deep inside the furnace is different. A loose inducer motor, a loose blower wheel or a cracked component can all rattle, and those need a technician to open the cabinet.

Popping, Ticking and Booming in the Ducts

Metal ducts expand as hot air flows through them and contract as they cool. That movement produces ticks and pops at the start and end of a cycle, and in most homes it is harmless.

A louder bang from the ducts every time the blower starts or stops is a different story. When a duct is too small or a return is too restrictive, pressure builds and flexes the flat sides of the metal like the bottom of a can. It isn't dangerous, but it is a sign the system is fighting for air. A blocked return, a clogged filter or closed registers can make it worse, and duct changes sometimes solve it for good.

Humming and Buzzing

A low, steady hum is normal. The blower motor hums, and so does the transformer that powers the controls.

A loud buzz, or a hum that gets noticeably louder than it used to be, often points to an electrical part under strain. A failing capacitor can make the blower motor hum without starting, and a contactor or relay can buzz when its contacts are worn. A motor that hums but doesn't turn will overheat if it is left powered. These are all high-voltage parts, so the fix belongs to a licensed technician.

Whistling and Whooshing

A whistle usually means air is being forced through a gap smaller than it should be. A badly clogged filter, a filter that is the wrong size and leaves a gap, a slot left open where the filter slides in, or a lot of closed registers can all cause it. Check those first. If the filter is clean and the registers are open, the ducts or the return may be undersized, which is something a technician can measure.

Clicking and Gurgling

A single click when the thermostat calls for heat, and another when it is satisfied, is the sound of relays doing their job. Repeated clicking with no flame is the furnace trying and failing to light, and the steps for that are in our article on a furnace not turning on.

High-efficiency models produce condensation while heating, and a gurgle can be that water moving through a drain trap that is partly blocked. If you also see water around the furnace, the drain needs clearing.

What to Note Before You Call

When a sound shows up, write down when it happens: at startup, while the blower runs, when the burners go off or all the time. Note where it seems to come from, the furnace, a vent or the ceiling. If you can, record a short clip on your phone. A sound that only happens once a cycle is easy to miss during a visit, and a recording lets a technician hear exactly what you heard.

Many noisy-furnace repairs are routine, such as a blower motor, a belt, a capacitor or cleaned burners, and they extend the furnace's life rather than signal the end of it. The same blower parts are behind many AC sounds too, which our article on AC noises covers for the summer side.

Getting the Noise Looked At

If your furnace has started making a sound it didn't make last winter, Dr. Cool can track it down. Same-day heating repair is bookable through our website or at (702) 873-1800, and for a booming startup or grinding metal, switch the furnace off and use our 24/7 emergency line. A yearly heating maintenance visit catches dirty burners and worn bearings before they get loud.