Forced air furnaces are usually fairly quiet if they are installed properly. But age and wear can eventually make them noisy and annoying. Noise from the furnace will travel through the pipes and be heard throughout the house.
Here are some of the more common noises coming from the furnace, and what is causing them. Quite often there is an easy DIY fix. Sometimes you are better off calling a professional to ensure proper repairs.
Furnace, Plenum, Filter, Air Return, and Outside Air Intake
6 Furnace Noise Makers
Although most furnace noise will come from the furnace itself, make sure you also check out other parts of the system. For more tips on quieting the entire system please see our article How to Quiet a Noisy Furnace Blower.
1) Furnace Noise Caused by the Blower and Motor
Furnace Blower and Motor
Some of the more common causes of noises coming out of the furnace blower compartment include:
- Blower Motor Bearings. Wearing out, or worn out, motor bearings will produce a squealing or whining noise that will come out of all of the vents. Blower motors–and bearings–should last for up to 30 years. Lack of lubrication (on older motors) or out of balance squirrel cage will reduce their lifespan. If you have the ability, change the bearings yourself. They are very inexpensive. Or you could just replace the whole motor.
- Broken Blower Wheel. Although rare, squirrel cage blower wheels can break–giving you a loud metal scraping sound. Turn off the furnace immediately. Blower wheels are fairly simple to replace, and inexpensive. (It is almost impossible to repair broken blower fins. And usually not worth the trouble.)
- Out of Balance Wheel. Out of balance wheels tend to sound like out of balance washing machines. This is also a rare occurrence. but can happen if a balancing weight falls off the wheel, or something is sucked into it. (Which can happen if you change the filter when the furnace is running.) If a weight has fallen off, you are better off replacing the blower wheel.
- Loose Mount. As you can see from the picture, the motor and blower wheel are hung from the bottom of the furnace proper. If you begin to hear rattling sounds, it is possible that the bolts are getting loose. Although the space is a little tight, you should be able to get to all of them to ensure they are tight. (If it keeps happening, lock washers or Loctite will keep the nuts from loosening up.)
2) Furnace Noise Caused by the Burners
Loud bangs, booming sounds, or an ignition whump are likely caused by dirty burners. Dirty burners cause delayed ignition, so there can be a build-up of gas before the gas lights. This is a dangerous situation and should be looked at by a professional PDQ.
3) Furnace Noise Caused by the Heat Exchanger
A clicking sound from your furnace could be signaling a cracked heat exchanger. Years of heating and cooling (expansion and contraction) can crack the metal. Every time the furnace fires up the cracks will start clicking as they expand. Cracked heat exchangers are one of the main culprits in spreading carbon monoxide into homes. Have your cracked heat exchangers replaced immediately.
4) Furnace Noise Caused by an Air Leak
The large metal cap on top of the furnace is called a plenum. It is screwed onto the furnace to collect and distribute the warm air. Loose screws will allow the air to leak out producing an airy whistling sound. You can fix this by tightening the screws, installing a foam compression weatherstrip between the furnace and plenum, or sealing the gap with duct tape.
Another possible location of air leaking is metal pipe joints. This can be easily fixed by replacing the duct tape around them.
5) Furnace Noise Caused by the Transformer
If you hear a humming sound coming from the furnace, it is likely caused by the transformer. Most of them will hum quietly while the furnace is running, but you should not be able to hear it. When you can hear the hum while standing beside the furnace, it is time to change it. Or call a furnace technician to check it out or change it out. Murphy’s law says that it will quit on the coldest night of the year, just after all the shops are closed.
Transformers are not terribly expensive and they are pretty easy to replace. Just make sure you turn off the furnace breaker before doing any work on it. There are quite a few wires to deal with–some of them carrying 120 volts.
6) Clicking, Humming, Vibration, or Squealing Noises
Some of the noises you hear sound the same but may be caused by different parts of the furnace.
- Clicking. Not only will the transformer hum loudly as it is getting towards its end of life, it may not start the furnace. In that case you will likely hear clicking coming from the furnace as it tries to start. The clicking usually means that there is not enough power being transferred through the transformer.
- Humming. You may hear a humming noise coming from the blower motor if the capacitor is worn out. Capacitors are small can shaped devices that store enough power to give the motor its initial kick. The hum is from the motor trying to start. Capacitors cannot be repaired. They must be replaced. (Note: You may also smell a burning electrical-type odor.)
- Vibration. Vibrations, or a low pitched vibration-type noise is most likely caused by inadequate anchoring of the furnace. Whether your furnace is sitting on the floor, bolted to the floor, or strapped to the wall, it can be loose enough to vibrate. Tightening the anchors will probably solve the problem. But for an even quieter furnace, insert anti-vibration pads between the furnace and floor or wall.
- Squealing. High pitched squealing noises almost invariably are produced by the belt between blower motor and squirrel cage. Some of the potential causes to look for are loose belt, misaligned pulleys, worn belt, loose pulleys. These are all simple, inexpensive repairs, adjustments, or replacements. With a little help from your owners manual.
Furnace Noise Caused by Peripheral Components
Although the furnace (usually meaning the motor or blower) gets the blame for the sounds you hear coming from the vents, noise can come from elsewhere. There are many other parts of the heating system that might be creating noise.
- Plenum. Not only can a plenum make whistling noises if it is not attached properly, it can make a kind of deep bonging noise if the large sections of steel expand and contract too much. This is usually not a problem, but occasionally the plenum is designed incorrectly, or it is forced into a position that does not allow for movement.
- Return Air. Return air ducts are also made of large wide flat pieces of galvanized steel that may expand and contract making the same bonging noise. (Note: This is not very likely because the return air is usually cool enough to prevent expansion.)
- Pipes. The pipes carrying warm air to each room can make noise in a number of ways. Including expansion and contraction, air leaks at the joints, blowing foreign objects around in the pipe. Expansion is only a problem if the pipe has nowhere to go at the end of a run, and is popping and banging as it tries to move sideways. Take it down and cut an inch or two off of it. Air leaks almost invariably occur from the pipe joints. Clean off the old duct tape and replace with new duct tape. Keep the insides of your pipes as clean as you can and hire a furnace cleaner once a year.
- Hangers. Almost all warm air pipes will be hung from metal straps or rest on metal cross braces. It is possible that you may hear metal scraping sounds from the interaction between hangars and pipes.
- Vent Covers. Loose vent covers can make a rattling sound if the air pressure is strong enough to make them move. The noise happens most often with plastic vent covers because they are quite light, and move easier than metal ones.
- Filters. You may hear a whistling sound coming from your heating system. This sound is quite often the result of restricted airflow caused by a plugged, or partially plugged furnace filter. The air is trying to get around the filter because it can no longer pass through it. (Note: Most furnace manufacturers recommend changing filters every 90 days.)
For more information please see our article Why is My Air Vent Making Noise?
New and Used Furnace Filter. One on the left is 6 weeks old.
When to Call an HVAC Professional
When to call a professional is usually a fairly easy decision, especially if you are honest with yourself. I will tackle quite a few repairs on my heating system. But I am willing to admit what I don’t know, and stay away from those things. (Replacing a motor and squirrel cage–absolutely. Changing burners–not likely.)
It is probably worthwhile to get a professional inspection if you have no idea what is causing the noise, the inside of your furnace looks like the Enterprise drive system, or you do not feel capable, or have no interest. The following picture is of the insides of my furnace. The only thing I might consider changing in there is the transformer on the back wall. All else I am happy to leave to the local company that installed it.
Power and Fuel Lines of a Natural Gas Furnace