Your furnace blower motor can make a variety of noises when it starts. Noises can indicate a range of problems – or be nothing at all to worry about.
If there is a problem, it could be something minor like a loose belt or screw, or something more serious like a worn-out blower motor or something very dangerous like a cracked heat exchanger, which is rare, so let’s not go there right now.
Whatever the issue, it’s best to shut your furnace down until you or an HVAC technician can diagnose and solve the problem.
Below we’ll go through some of the common noises a furnace or blower motor can make, the possible causes of a specific noise, and what can be done to fix it.
Reasons for Furnace Blower Motor Making Noise When Starting
If you have HVAC experience and are comfortable working on your furnace, you’ll find some things below that you can try to resolve. Once you realize that a furnace is a fairly straightforward piece of equipment, working on it becomes less daunting.
Most of the fixes below require you to remove the blower assembly from the furnace which is an in-depth HVAC project and not suitable for most homeowners.
Blower Motor Make a Humming or Buzzing Sound
A humming or buzzing sound is a common blower noise homeowners notice. Sometimes the blower fan speed just needs adjusting to solve the problem, but the blower motor could be making the noise for several more serious reasons.
Squealing or Grinding – A Lack of Lubrication
Blower motors need lubrication to run smoothly and quietly. With newer furnaces, the blower motors are permanently lubricated and factory sealed, but on older furnaces, the blower may have oil ports and require annual lubrication. Check with your furnace manual to see if you can lubricate the motor yourself. Or look for ports or openings that seem to be there for adding oil/lube, and give them a squirt of quality appliance lube.
Dirty Blower Motor
A dirty blower motor can make a humming noise. This can happen when the furnace air filters aren’t changed regularly allowing dirt and dust to get inside the unit. Remove the service panel to your furnace and locate the blower. If the blower appears to be excessively dirty, it needs a thorough cleaning.
Misaligned or Bent Blower Wheel of Shaft
A misaligned or bent blower wheel or shaft is a less common issue but can also cause a humming noise. If this is the case you will need to remove the blower assembly and replace the damaged part.
The hum could also be caused by a failing motor that needs to be replaced, a faulty transformer, or a bad draft inducer motor. In these cases, an HVAC contractor should be called to inspect your furnace, pinpoint the issue, and fix it.
Blower Motor Makes a Screeching Sound
This is one of the more annoying sounds – and one that alerts you to the fact that something isn’t right. Here are causes and cures.
Loose or Damaged Belt
When the blower motor makes a screeching or squealing sound, it likely means a bad belt is the cause. Belt-driven blower fans are common in older furnaces and wear down, stretch, and crack over time. If the belt is making noise, is slipping, or shows signs of cracking, it’s time to replace it. Here is a video featuring blower motor belt replacement.
Issue with the Motor Bearings
A screeching sound can indicate a problem with the bearings in the blower or inducer motor. Bearings allow various parts of the motor to move together without creating friction between the parts, so your fan motor can turn smoothly and quietly.
Bad bearings are often caused by dirt, overheating and lack of lubrication, especially in an aging unit.
If you allow the bearings to wear down all the way, the motor will seize or burn out. Have the problem promptly repaired by an HVAC technician who will replace the bearings before damage to the motor occurs.
Pro recommendation: You should consider replacing the whole motor assembly rather than just replacing the bearings. The price will be close to the same, and you’ll have a worry-free blower for years to come. Talk with a furnace technician about your options.
Blower Motor Makes Scraping or Grinding Noises
Two of the most common reasons for scraping or grinding noises are a loose blower wheel or a broken blower motor mount or bracket. The blower wheel is attached to the blower motor by a screw. Occasionally the screw will come loose and allow the wheel to rub against the blower housing. This is easily fixed by tightening or replacing the screw.
A broken blower motor mount will also allow the blower wheel to run against the blower housing. Replacing a blower motor mount is easy if you are able to get to and remove the blower assembly. This is a DIY job on most furnaces.
Blower Motor Makes Thumping or Clanking Sounds
These sounds are usually the result of a loose or broken part inside the blower assembly. If you hear these noises, it’s a good idea to turn off the furnace until it can be inspected and repaired. If a part comes loose, it could easily damage other parts.
Popping or Banging Noises When the Blower Motor Turns On
There are a couple of reasons you might be hearing this in your furnace.
Banging or popping noises that you hear about the time the blower turns may indicate delayed ignition of the gas. When gas can’t ignite right away it builds up in the combustion chamber. The banging is caused by mini explosions when the gas finally ignites. Delayed ignition can be caused by a variety of issues including too much air mixing with the gas, dirty or misaligned burners, moisture, rust, a dirty pilot light (on an older furnace), or a malfunctioning hot surface ignitor (on a newer furnace). Delayed ignition can be a serious situation, cracking the heat exchanger or even causing a fire in your furnace space, so shut down the furnace and call an HVAC professional if you think this could be the cause of the issue you are researching.
Your blower turns on and begins to move hot air through metal ductwork. Thin sheet metal ductwork can get very cold in the winter and as it is heated it will expand making popping or banging noises. While this is not a serious problem, the noise may be bothersome. If you choose to do either of the fixes below, your investment could be quickly recouped in lower energy bills.
Insulate Your Existing Ductwork
Insulating your ductwork will go a long way to solve the popping noises. If you insulate your ductwork, make sure you seal them first to plug all of the leaks.
Blower Motor Makes a High-Pitched Rattling Noise
Here is the rare case that should be immediately addressed.
A high-pitched rattling noise just before or at the time when the blower turns on almost always means the heat exchanger is cracked. A cracked heat exchanger is a serious issue and can allow carbon monoxide leaks into your home. The furnace should be shut off until an HVAC professional can inspect the furnace.
Blower Motor Makes a Whistling Sound
A whistling sound may be caused by a dirty or clogged air filter. As your blower turns on, air is pulled through your filter. If the air flow is restricted, a whistling noise may result as the air moves around the filter. Check your air filter, and change it if it’s dirty. Airflow may also be restricted in fresh air intake vents. Check these vents for animal nests, leaves, and snow and ice.
Rumbling Noise When the Blower Motor Turns On
If your furnace makes a low rumbling noise when the blower turns on, it could mean you have a dirty burner. Soot builds up on the burners during normal operation and should be cleaned annually. When soot builds up it blocks the air flow needed for combustion causing a rumbling sound. Check the burners and if they are overly sooty, clean them or hire an HVAC professional or a cleaning and tune-up.
Cleaning furnace blowers isn’t a simple job, but doing it yourself can save money when you do the job correctly. Here’s the Pick HVAC guide to Cleaning Furnace Burners.
Here are additional signs that you need to replace your furnace blower motor.
And here are additional ways to quiet a noisy blower motor.