October 29th, 2021

Hey everyone, it’s Cosmo again! We dogs have an excellent sense of hearing. Not only can we hear higher frequencies than humans, but we can also hear sounds from greater distances. There are some sounds we love to hear, like when a person is fixing a meal for us, but there are other noises we don’t like – and you shouldn’t either! Here are some potentially problematic sounds you might hear coming from your furnace and what might be causing them.

Knocking or Banging When the Furnace Turns On

This sound is most likely coming from either your ductwork or burner. Let’s explore each of these possibilities:

Oil-Canning Ductwork

If there’s a weak spot in your ductwork, pressure can change that area’s shape. Increased pressure will push the ductwork’s walls outward, and they’ll return to their original form when the pressure decreases. This can lead to further ductwork damage if left unaddressed, but fortunately, it’s not too difficult for a professional to fix. The way an HVAC technician can fix this is by identifying the weak spot and either replacing a section of ductwork or reinforcing the weak section with a thicker piece of metal.

Furnace Ignition Roll-Out

Ignition roll-out is relatively rare, but we’ve seen it, and we know how startling the noise can be. The sound comes from a small explosion that occurs when the furnace starts up, which is caused by an excessively high concentration of gases within the combustion chamber. Ignition roll-out indicates that instead of exiting the home via the flues and up through the vent, combustion gases are lingering within the furnace and giving too much fuel to the fire.  

If ignition roll-out occurs in a newer furnace that is equipped with a roll-out sensor, the sensor will trigger the system to automatically shut down. If you suspect that your furnace is experiencing ignition roll-out, call in a professional. With fire and gases involved, it’s much too dangerous to try to fix by yourself. 


The most likely source of humming from a furnace is the blower motor. The issue could be a lack of lubrication, which is why lubricating moving parts as needed is part of every Heating Precision Tune-Up we perform. Another possibility is that the motor is experiencing low airflow due to dirty filters, especially if the humming tends to occur when the motor is slowing down. 

Other possible reasons for humming include a failed capacitor and loose components or ductwork. With so many possibilities, it can be tough to diagnose the problem if you’re not a trained HVAC technician. 


When furnaces buzz, the noise often originates from the furnace’s transformer box, located inside the furnace. The job of a transformer is to take the incoming power and use it to give the furnace its required voltage. When the transformer is starting to die, or if it is not securely mounted to the furnace’s interior, it makes a buzzing noise. 

Blower motors and capacitors are other possible buzzing noise sources, occurring when these components need replacement or soon will need replacement. 


Furnaces never whistle a happy tune! A whistling furnace could be a sign of gaps in your ductwork. If there’s a small hole near the blower, between the duct and the furnace, air will create a whistling sound as it escapes. This will also occur if the furnace has a clogged filter, restricting airflow and causing the furnace to suck air from gaps in the ducts. If this is the case, replacing the filter will serve two purposes: eliminating the whistling noise and helping the furnace run more efficiently, with less stress on the fan motor. Another duct-related issue that can cause whistling is improper sizing. If the furnace is too large for the ducts or the ducts are too small for the furnace, insufficient airflow will cause whistling in this case as well. 

Vibrations & Motor Noise

Picking up bad vibrations from your furnace? A clogged filter might be to blame. If the return side airflow is restricted, it can cause the air pressure to decrease, making the furnace shake. Worn motor mounts and out-of-balance fans also cause excessive vibration. Another source of noise is from motor bearings that are worn out or getting damaged.

Sound is subjective, and some of these sounds might be difficult for you to describe or diagnose, but that doesn’t mean you can’t trust your ears. If you think something doesn’t sound right, calling a professional is the best thing to do. Even if there isn’t a mechanical problem and the issue is as simple as tightening some loose parts or adding some Styrofoam to muffle the noise, you’ll end up with a quieter furnace and greater peace of mind. If the noise is caused by a mechanical issue that calls for replacement parts or furnace repair in Mercer County, NJ, it’s best to leave the diagnosis and the repair job to professionals. At SG Heating & Air Conditioning LLC, our technicians have decades of combined experience, meaning that whatever issue your furnace has run into, chances are we’ve seen it before and know what it’ll take to resolve it. 

To get your furnace running and sounding like its old self again, or to get in touch with an expert furnace installer serving the Princeton, NJ area, call us today at 609-448-1273!

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