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Furnace Making Clunking Noise? 4 Common Causes (+ Fixes)

A clunking noise coming from a furnace can be a cause for concern. The sound often comes from metal parts rubbing against each other. When a furnace is making a clunking noise, you should investigate the sound to determine what’s causing it. 

Your furnace is making a clunking noise because of expanding metal panels, loose components, a cracked fan belt, or worn-out motor bearings. You can fix the problem by inspecting the furnace and ensuring that everything is mounted properly or replacing the fan belt and bearings.

If your furnace is making a consistent clunking noise, this article will help you find the reason for the sound. You’ll also learn how to fix each issue. 

Metal Panels Expanding and Contracting

There are certain times when a clunking noise coming from the furnace would be considered normal. Maybe the sound is made by the metal housing, in which case you don’t have to go through tedious troubleshooting steps as there’s nothing wrong with the furnace in the first place.

Several metal panels form the furnace housing. Additionally, you probably have metal ductwork, another potential sound source. As heated air is blown close to the panels, they will expand. Once you turn the furnace off, the panels will gradually cool down. This causes the metal panels to contract. 

Both of these processes can create a clunking sound. Contracting and expanding metal panels aren’t a problem. This shouldn’t happen continuously, only during the heating cycle and when the system cools down. Sometimes, however, the sound can be associated with a problem related to the ductwork in your HVAC system. 

How To Fix

If this is the reason for the noise, there isn’t much you can do about it. The metal expands in the presence of heat and contracts when the temperature drops. Thankfully, it isn’t something that you have to worry about.

However, you should first determine if this is the case with your furnace. Stand next to the furnace as it’s cooling or heating and listen closely to where the sound is coming from. Does the sound originate from the panels on the furnace or perhaps the ducts? If this is the case, then it’s not a problem you should spend any more time on.

But if the sound comes from the inside, you have a bigger problem and should open up the furnace to investigate further. 

Loose Components Inside the Furnace

When it comes to problems that cause the furnace to make a clunking sound, you want to look for loose components. For example, when a motor isn’t mounted correctly, it can continuously bump against metal parts in the furnace. When this happens, you’ll hear a loud, constant clunking sound. 

Other parts of the furnace can also come loose and create a similar sound. You need to inspect the inside of the furnace to determine if loose components are causing the clunking.

How To Fix

If you think that there are some loose components in the furnace, here’s what you need to do.

  1. Turn the furnace off at the circuit breaker.
  2. Remove the panel that covers the main compartment of the furnace.
  3. Look for loose components inside the furnace. Make sure to check each part. Gently wiggle the motor, burners, fans, and other parts. 
  4. If you find loose components, fasten them. Sometimes, it’s a loose screw; other times, it’s a broken piece of plastic that you can fix with some glue.

If you find a component that’s completely out of place, you need to find where it goes. Use the owner’s manual or look at pictures of your furnace online to identify it.

Cracked Fan Belt

When all components are fastened yet you still hear a clunking noise, it’s time to look for damaged parts. Start with the fan belt. The blower motor drives the belt and, as it turns, it rotates the fan. 

If a fan belt is cracked, then a part of it can constantly push against metal areas of the furnace. This can also be the source of clunking noise. As the belt cracks and tears, the sound will become louder and more frequent. A loud squeaking or clunking sound is a sign of a significant problem. So, attend to the issue while it’s still a subtle noise. 

How To Fix

There isn’t a way to fix a cracked belt. Instead, you will need to consider replacing the entire fan belt. Take note that there are multiple fan belts in a furnace. If you suspect a cracked fan belt, you should investigate all fan belts. Here’s how:

  1. Switch off power to the furnace and open it up. 
  2. Identify the location of all fan belts used in the system. 
  3. Check the fan belts by visually inspecting the component. See if you can find cracks. 
  4. If there are cracks, replace the fan belt with an identical one. 

The video below offers a guide on replacing a faulty fan belt:

Worn-Out Motor Bearings

After checking the previous potential causes and finding no evidence behind the clunking noise, you should move on to investigating the motor bearings. 

Bearings ensure the fan rotates smoothly and blows hot air through the ductwork. When a bearing is worn-out, it can cause a clunking sound. For example, if a part of the bearing breaks loose, the broken pieces can rub against metal parts as they move. 

How To Fix

You shouldn’t fix broken bearings; replace them instead. Take a look at the furnace’s bearings to determine if they’re broken, have cracks, or seem worn out.

Just like fan belts, there’s more than one bearing in the furnace. Each motor has one or multiple bearings that rotate the fan, along with the fan belt.

Replacing the bearings does more than just eliminate the annoying clunking noise. The new bearings will also preserve the motor’s functioning, as worn bearings cause metal-on-metal movement, leading to damage in the long run. 

Summary

A clunking sound can be caused by contracting and expanding panels, but there are also times when the clunking signifies loose parts or other problems. 

Hiring a qualified contractor to work with HVAC systems is the most effective way to take care of this problem. The expert can analyze your furnace and see why the clunking noise occurs.