Your furnace is made of many moving parts—so you might hear some noises associated with any working mechanical thing. However, there’s a difference between which noises are “okay” and which may indicate a problem. If you’ve noticed any strange sounds coming from your heating equipment, take a look at this list to see what they might mean.
Scraping → indicates something is wrong with your fan. You should turn off your furnace right away if you hear a scraping noise to keep the motor from burning out. It may be there’s a foreign object blocking the blower, or one of the fan blades may be warped or broken. Either way, it’s best to address it right away to avoid a bigger repair.
Thumping → indicates something is out of balance. Just like when your washing machine thumps when the load is unbalanced, your furnace may be thumping because a motor or blower is out of balance. Address it now to avoid big problems down the road.
Rumbling → is a pretty common sound for a gas or oil-fired furnace when it’s slight and doesn’t last more than a few seconds. However, if the rumbling is loud, or lasts an unusually long time, it could mean there’s a serious problem with your blower. In this case, turn off your furnace and call for service.
Rattling → isn’t usually a big deal. It often indicates the fasteners of the metal housing over your unit have become loose. That’s an easy fix. However, if there is rattling or clanking coming from the inside of your unit, that’s something else altogether. Some debris may have lodged inside and is clanking around—if you can see something, turn off the furnace first before attempting to remove. If you see that debris has caused any damage to your compressor or fan, call for service.
Popping → is a common noise when furnaces get turned on at the beginning of the cool season. You may hear a series of loud pops when the fan starts and stops because the sheet metal in your ductwork is responding to pressure. It pops in when there’s negative pressure from the fan, then pops out when the fan shuts off and releases that pressure. If the popping noise persists or increases, it could indicate you have undersized or flimsy ductwork, too many closed vents, or a clogged air filter.
Hissing → caused by escaping air, it could indicate you have an air filter that wasn’t properly installed, or perhaps is the wrong size. It might also mean you have too many closed vents in the house, or furniture is blocking air flow. If the sound becomes very loud, it may mean there’s a leak somewhere in your ducts. It’s worth a duct inspection to see if you’re losing a lot of expensive heated air to the walls and attic of your house.
Whistling → is not a good noise when it comes to your HVAC system. Whistling could mean there’s a refrigerant leak in your condenser or another piece of equipment that’s causing internal pressure. If you hear whistling, you should call for a service technician to come take a look.
Generally a strange noise coming from your furnace means it’s time for a tune-up at the very least. If any noise becomes loud or disruptive, don’t ignore it and hope it goes away—contact a trusted local HVAC company who can send out a trained technician to locate the source of the noise and correct it. It’s best to take care of a small problem before it develops into a big repair.