Why Is My Furnace Buzzing? Sources and What To Do

A noisy furnace is annoying, but it can signify that something in your furnace is starting to fail. If you are wondering why your furnace is buzzing, we have the answers.

The most common sources of a buzzing sound from a furnace include:

• The blower fan
• Motor bearings
• The draft inducer
• The Belt (certain models), or
• Expansion of the ducts.

You should check each of these so they can be repaired before your furnace stops functioning properly.

The good news is that a homeowner can do some of these repairs. A little detective work can help you decide if you want to tackle the job yourself or call a furnace repair person to do the work for you.

Check the Blower Fan and Motor

If you hear a rattling noise, the problem could be coming from your blower fan. The blower fan’s job is to send, or blow, the warm air created by a heat exchanger.

You will recognize the heat exchanger by a series of interconnected coils or tubes. The shape of the coils is determined by the fuel source and model of your furnace. The coils are usually located right above the heat exchanger.

The purpose of the heat exchanger is to create the heat. However, without a blower fan to push the warm air through the ductwork, the heat would stay in the furnace instead of warming your house.

Sometimes the fan is called a blower wheel because it is round like a wheel. It’s also called a squirrel cage because it looks like a giant exercise wheel for a mouse or rat.

The blower wheel attaches to the blower shaft with a set screw. If this screw is loose, then the blower wheel will shake and make a rattling noise as it spins. The set screw is usually located in the center on the outside of the wheel. Check the screw and make sure it is properly tightened.  

The mounting bolts that attach the motor to the blower fan housing can also become loose. If the set screw is tight, check the mounting bolts attached to the blower fan and tighten it if necessary.

If you hear a grinding noise, the blower fan is likely scraping against metal. Check to see if something is stuck inside the fan and is hitting the blades.

Check the Motor Bearings

If you hear something that sounds similar to a loud idling truck, then the fan motor’s bearings could be worn down.

Some older motors can be lubricated to reduce the noise, but newer ones cannot. Most blower motors are in the $100-200 range. Replacing one is not difficult, but it is time-consuming, which is why the typical labor cost for replacement is around $250 (source).

Check the Draft Inducer

The motor that runs a draft inducer is smaller, and therefore it has a higher-pitched sound than the blower fan.

The draft inducer pulls air and gasses out of the furnace and through the flue or chimney. When a furnace starts up, the draft inducer blows out any gas left over from the last time the furnace ran. Doing this helps keep soot from forming in furnaces and increase the system’s efficiency.   

The blower fan of the inducer starts running 30 to 60 seconds before the furnace burners turn on. If the draft inducer does not turn on, the furnace will shut down, and you will be unable to start it again.  

A draft inducer can also shut down a furnace for several other reasons, including:

• A component in the rubber tube has gone bad
• The flue is blocked
• The pressure switch is broken

Most draft inducer motors cannot be repaired, so it would need to be replaced. And since a broken inducer will prevent your furnace from working, it should be replaced before it breaks.

The average cost of a draft inducer motor is around $175, and a contractor will charge you around $80-100 to install it. If you know your way around a multimeter and understand voltage, resistance, and ohms, you can troubleshoot your inducer motor. 

This video can lead you through the checks:

How to tell if an Inducer Motor is BAD: Explained Step by Step

If you do not feel confident in your ability to check for this and do not want to spend $175 needlessly, call a heating specialist.  

Check the Belt

If you hear a squealing sound, then you might have a problem with the belt.  

Older furnaces use a belt-drive motor. The belt runs from the motor to the blower fan and turns it. These belts can get worn down and begin to slip for several reasons:

• They stretch over time
• Dirt and grease 
• They get out of alignment

First, clean the belt using some soapy water. Then check the alignment between the motor and the fan. If the belt looks worn, then you should replace it.  

Replacing a belt can be done in under half an hour if you have a socket set and screwdriver.

1. Turn off the furnace at the breaker box. If you set the fan to On, the motor should not come on.
2. Remove the panel on the furnace.
3. Loosen the motor enough to push the motor to the blower enough so the old belt can be removed.
4. Put the new belt on and tighten the motor.
5. Check the tension of the bolt. If it gives more than half an inch, then adjust the motor bolts so that the tension is correct.
6. Give your furnace power and make sure the belt works correctly. And do not forget to put the access panel back on.

Check for Duct Expansion

Should you hear loud or metallic pops when your furnace runs, then you probably have duct expansion.  

Although flexible ducts are commonly used due to their lower cost and ease of installation, many systems have metal or rigid ductwork. Metal ductwork lasts longer and is less likely to have mold growing in it.  

As hot air blows through the duct, the change in air pressure causes the sheet metal to expand and contract, and this creates the popping sound. Thinner metal is more susceptible to expansion. Rectangular ducts are more likely to pop, and circular ducts are the least likely.  

You could replace the ducts with thicker gauge metal, although that would be expensive. Another option would be insulating the ducts, which is also not cheap. In some cases, the contracting metal hits the hangers, and it’s also possible that an expansion collar could stop banging that occurs at the joint closest to the furnace.  

Delayed Ignition

The burners in your furnace can become gummed up over time. Blocked burners won’t ignite properly, so there is a delay until they start up. This delay can last several minutes.

• To see if this is happening, you will need a second person. Have one person turn the furnace on while the second person waits at the furnace to see how long it takes for the burners to ignite.

Delayed ignition needs to be repaired before your heat exchanger cracks. When you have your annual furnace check-up, the technician should clean and check the heat exchanger.

Unless the problem is delayed ignition, the banging sounds, annoying as they are, won’t affect how well your furnace operates.  

Bottom Line

Even the quietest furnace will make some sound. You want to watch out for changes in the sound of your furnace. If you hear a rattling noise, check the blower fan.

A sound like an idling truck suggests a problem with the blower motor, and a squealing sound could be the belt. It’s best to prevent these problems by having your furnace serviced once a year. 

Related Reading:

• Furnace Flame Goes Out When Blower Comes On
• Insulation Around Furnace Exhaust: Important Do’s And Don’ts