What is that noise coming from my furnace?

December 06, 2018

You've been using your heat more as the weather gets colder, and you might notice some noises that seem to coincide with the furnace kicking on. These might be nice quiet sounds, like the low steady hum of fans and motors, or air whooshing through your vents. However, there may be other noises that are not in the “nice steady hum” category, and they may indicate a problem that needs investigation.

If you hear a scraping noise -- Let's just say scraping sounds are not good sounds when they're originating from your HVAC. A scraping noise usually means something is wrong with the fan blades and/or motor. If you hear something that sounds screeching or scraping, turn off your heating equipment right away. A foreign object may be obstructing your blower, or one of the fan blades may be warped or broken. In either case, it can cause your motor to burn out. You'll likely need to call a technician to inspect your furnace and correct the problem.

If you hear a thumping noise -- Things that go thump in the night or any other time means something is out of balance. Thumping sounds that come from your furnace could indicate that a motor or blower wheel is out of balance. This could cause big problems for your mechanical equipment down the road. It's best to have an HVAC professional take a look - and fix a small problem before it becomes a big one.

If you hear a rumbling noise -- A low rumble is a normal sound when your gas or oil-fired furnace kicks on. However, if that rumble is louder than usual - or lasts longer than usual - this could indicate a very serious problem with your burner. If you hear a loud or long rumbling noise coming from your gas or oil furnace, turn off your equipment and call for service.

If you hear a rattling noise -- A rattling noise could mean that fasteners to the metal housing of your heating unit have worked loose. That's not usually a serious issue, and those fasteners can be easily tightened. However, if the rattling noise is coming from inside the unit, that's another matter altogether. You should call a technician to see if the problem is serious. Sometimes twigs and other outdoor items can get inside the fan of an outdoor unit, causing a little clanking or rattling around. If you can see this is the problem, you can just switch off the power and remove the debris. If you see the debris has somehow caused damage to the compressor or fan, call a technician.

If you hear a popping noise -- Popping sounds are quite common when your turn on your furnace at the beginning of the heating season. This is usually the sheet metal in your ductwork responding to pressure -- popping one way in reaction to the negative air pressure of the fan, then popping back when the fan shuts off and the pressure releases. It's not usually a worrisome noise unless it becomes very loud or very persistent. In that case, you might want to have your ductwork inspected.

If you hear a hissing noise -- If you're hearing a hiss, you're likely hearing escaping air. This could be caused by an improperly installed air filter, or one that is the wrong size. Often having too many closed vents, or having furniture blocking air flow can be the cause of a hissing sound. If the hisses become very loud, it could indicate there's a leak somewhere in the ductwork - and that means a lot of expensively heated air is escaping where it can't be used. Again, if this is the case, you might want to have your ductwork inspected.

If you hear a whistling noise -- Whistling noises fall into the category of “not good” when it comes to your HVAC equipment. This could mean there's a refrigerant leak in your condenser, or some other problems that's causing internal pressure. If your furnace is whistling at you - call for a service technician.

Heating and cooling units are mechanical equipment, and so they make noise. Not all that noise is bad, and even the occasional strange noise may merely indicate that it's time for a tune-up. However, any loud, disruptive, or continual noises usually mean there's a real problem and you should contact a trusted local HVAC company like A.J. Perri to come and take a look. Trained technicians will be able to tighten and lubricate parts, replace belts and filters, and take care of small problems before they become big repairs - or possible dangers.

Last Updated: September 13, 2023