Why Is My Furnace Making a Loud Humming Noise?

Stacy Randall
by Stacy Randall

It’s perfectly normal for your furnace to be making a little bit of noise while it’s running. Most appliances emit a low buzz or a soft hum while they are in use. However, if your furnace is making a loud humming noise, you might need a maintenance specialist.

A furnace can hum loudly if the blower motor or transformer is outdated or damaged. You can replace both parts to make your furnace run smoothly. Turn off the furnace, add lubricant to the motor, and run it to see if it is quieter.

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The Most Common Reason for Furnace Humming: The Blower Motor

The most common reason for a furnace that is humming is the blower motor. Your blower motor may be humming for several reasons, including inadequate lubrication or an amperage problem.

If your blower motor is not lubricated enough, this an easy fix, simply follow these steps:

Step 1

First, make sure to shut off your unit. Use an oil exclusively made for blower motors; all-purpose oil is not ideal.

Step 2

Locate the oil ports in your blower motor, use your manual for assistance if needed.

Step 3

Apply two to three drops of oil into each port. Don’t overdo it as using too much oil can lead to a host of other issues.

Step 4

If the shafts on your blower motor have additional oil ports, repeat step three with these ports.

If your furnace doesn’t come with a self-oiling system, it’s imperative to lubricate your blower motor once a year. If lubricating the blower motor did not solve the problem, you’ll need to call an HVAC professional to come to check it out.

If your blower motor is humming as it slows down, there may be an amperage problem. You’ll likely need to replace your blower motor soon.

It Could Be the Transformer or Loose Equipment

Less common than an old blower motor is a worn-out transformer. If your transformer is humming, you will likely need to replace it soon. Be sure to replace it before it dies, or you’ll have to deal with a frosty house in the winter!

The humming may also be a result of loose equipment in your furnace, which is a sign of poor maintenance. It’s vital to get any loose pieces fixed before your furnace fails you completely.

Another reason for persistent, loud humming in your furnace is loose ductwork. Over time, ductwork seams can loosen and start to vibrate with use, causing the unpleasant humming sound that you hear.

If this is the case, an HVAC repairman can take care of this problem by tightening seams, fixing air leaks, and stopping the vibrations.

The last potential cause of the annoying humming sound might be a failed capacitor. If you hear a hum but your motor isn’t turning, your capacitor has most likely gone out. You will need to call an HVAC expert to fix or replace the capacitor to ensure you have a working furnace.

How to Troubleshoot a Humming Furnace

Step 1

Turn off the furnace blower motor and let it cool.

Step 2

Lubricate and restart the motor according to the steps mentioned above. If this works, you’ve solved your problem! If not, continue to the next step.

Step 3

Using a motor meter, check your blower motor’s amperage draw. You can find the required draw amount on the plate of your motor. If the draw amount is higher than required, this is a sign your motor has been working overtime.

Step 4

Replace your motor; call an HVAC professional to take care of this for you depending on your level of expertise.

NOTE: The motor has likely gotten dusty and clogged due to dirty filters, making it difficult for the motor to do its job. Clean the filters regularly to prevent this from happening again.

Step 5

If your motor is not the problem, it may be your capacitor. Check to see if your blower motor turns when it is on or if it just hums.

Step 6

If the blower motor isn’t turning, it’s time to replace your capacitor. Again, use an HVAC pro for this step unless you have the expertise to handle it yourself.

Step 7

If replacing your capacitor didn’t help, your unit is most likely overheated; if so, reset the safety buttons on your furnace.

Step 8

If your blower motor is shut off and still humming, this is a sign of a pilot light issue. Readjust your pilot light to the correct level and see if the problem is resolved.

Step 9

If your blower motor is humming while in use, it is likely that your gas burners are dirty. Clean or adjust your burners to see if that makes the sound disappear.

Of course, if none of your own troubleshooting steps solve the issue, your best bet is to call in a pro. Contact an HVAC professional that can examine your unit and give you an accurate and thorough diagnosis of the problem and a plan to fix it.

Your furnace will make a lot of different sounds during its lifespan, but not all sounds are bad!

Some normal furnace sounds include:

  • Crackling. The metal parts of your unit are likely cooling down.
  • Pinging. This is the sound of air ducts moving and expanding.
  • Low Humming. This happens when the furnace burner is initially lit.
  • Chirping. This is a common furnace noise.
  • Rattling. Again, your system is cooling down. This may be a cause for alarm if it is happening often or is louder than usual. 

However, there are definitely some furnace sounds that are a cause for alarm.

Some not-so-normal furnace sounds include:

  • [b]Screeching. This usually indicates a problem with the motor or fan belt.[/b]
  • [b]Whining. This indicates a problem with your blower belt or blower motor. The shaft bearings may also need to be oiled. Call an HVAC professional for assistance.[/b]
  • [b]A Boom. This sometimes occurs when ducts expand and contract during different seasons. If a burning smell accompanies the boom, this could spell a bigger problem. You may have a dirty burner or faulty gas valve.[/b]
  • [b]A Clunk. This indicates a cracked belt. Call an HVAC professional for a replacement.[/b]
  • [b][b][b]Scraping. This indicates a problem with your blower wheel; it may be loose or broken, requiring tightening or replacement. You should turn off your furnace and call an HVAC professional to diagnose the problem. [/b][/b][/b]If your blower wheel is not loose or broken, you might need to replace the motor mount.
  • Rumbling. This could be a sign that fuel is continuing to burn even after the furnace has finished its cycle. This may be a problem with the burner or pilot light. You should call an HVAC professional immediately as this may produce carbon monoxide.
  • Loud humming. As discussed above, this could be a problem with the fan or capacitor. Call an HVAC professional to help with this issue.
  • Clicking. A click should begin and end a furnace cycle. However, if it continues throughout the cycle, it could be a problem with the compressor or control panel
  • Whistling. This is probably caused by a gap in the ductwork or a dirty filter. 

Related Questions

Is it normal for your furnace to make noise? 

When a furnace starts up, it’s perfectly normal for it to make noise! It will usually be a click as the furnace lights up followed by a low hum as it works. 

Can you soundproof your furnace room?

If your furnace room has doors and windows, you can soundproof them. Soundproofing curtains or barriers are available online and seal air gaps that leak sound. This will make your furnace’s regular noises less audible. 

Are modern furnaces quieter?

Many new furnaces are quieter than older models. Lennox brand models offer SilentComfort™ which makes their furnaces 50% quieter than competitors. These furnaces have high and low-speed settings. When in the low-speed setting, the company boasts the furnaces are very quiet. 

Are higher efficiency furnaces quieter?

Furnaces with a higher AFUE (annual fuel utilization efficiency) rating tend to have lower decibel levels. High-efficiency furnaces operate with advanced motors and other components that allow quieter operation. 

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Wrapping Up

If your furnace is making a loud humming noise, it may need its blower motor, transformer, or capacitor replaced. There may also be loose ductwork or equipment in your furnace that needs tightening.

To stop this humming noise, troubleshoot your furnace. Check to see if your blower motor, capacitor, or transformer need repair or replacement and tighten loose ductwork and equipment. If none of your attempts solve the problem, call a professional for help.

Furnaces make a lot of noise. Some noises, like chirping, low humming, and pinging, are normal with the use of your furnace. However, many noises are cause for concern and should be looked at by an HVAC professional. An expert can help you fix your furnace so you’re not left out in the cold!

Stacy Randall
Stacy Randall

Stacy Randall is a wife, mother, and freelance writer from NOLA that has always had a love for DIY projects, home organization, and making spaces beautiful. Together with her husband, she has been spending the last several years lovingly renovating her grandparent's former home, making it their own and learning a lot about life along the way.

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