A great deal of calls I received as a technician was clients concerned with strange noises coming from their furnaces. We will go over the more common ones in this article.
Ducts
The first noise complaint that comes to mind is a “popping” noise when the furnace fan starts. This sound can be quite annoying, especially at night. This was the case with this new furnace and ductwork installed (shown in the photo above). This sound is generally caused by the ductwork flexing when the blower is activated. If your ductwork is new, sealed, and didn’t make this noise until recently, check your air filter. When ducts are new and tight, and the blower is activated, it creates a change in pressure which can cause a point in the duct to pop in and out. Try checking your air filter first, it may be blocked, causing stress on your supply duct. If it is clean and making the noise, it is likely on the return duct. The client in this case just needed a new filter. In either case, you can have a duct sound dampener installed and solve the issue.
Inducer
The next that noise complaint which I run into is the inducer motor failing. If this is the case, the noise will be immediately when there is a call for heat. In a heating ignition sequence, the first thing to run is the inducer motor and the blower will be last. So by this logic alone, you should be able to deduce whether it’s the blower motor or inducer motor making the noise.
The photo above shows an example of an inducer on a furnace. This style in particular can be found on Carrier and Bryant units and incorporates a “floating” design using 3 bushings to keep the motor from vibrating. A common occurrence is the bushings wearing out and causing the motor to sag. This causes the fan blade to rub against the inside wall of the inducer assembly and very loud noise when starting and stopping only. That is due to centrifugal force taking over and leveling out the motor once the fan is spinning at max rpm. Bushings can be purchased and replaced very easily if this is your inducer. In most cases, however, it’s better to just replace the entire inducer assembly.
Blower
The last, common noise complaint I get is from the blower. There are a few different reasons why a noise could be coming from a blower. The first common reason is that some debris got sucked into the blower wheel, (shown in the photo above) usually a filter or part of a filter. This happens a lot when a very cheap, flimsy filter is used with no rigidity screening. These become clogged quickly and then just fold up. Next would be an unbalanced blower wheel. This will lead to a “wheel wobble” and will cause a bad vibration noise when running. Last would be failed motor bearings. If this the case, the sound will be a grinding noise. If all you hear is a “humming” sound, I’m sorry to inform you, but your blower motor or capacitor is shot and needs replaced.
Hopefully, this article has helped you to isolate the source of the noise in your furnace. For additional questions, feel free to drop us a line through our instant messenger or our virtual diagnostic tool.
