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

The furnace has several parts that are fastened with screws and bolts. As the air gets forcefully pushed through the system, these parts need to stay in place to prevent the air from escaping. A furnace making flapping noise is often a sign that some part has come loose. 

Your furnace is making a flapping noise because something is stuck in the vents, the ductwork or a panel on your furnace is loose, or some internal part is loose. Stop the flapping by ensuring that all components are fixed into place. Check for loose or broken screws, bolts, nuts, and brackets.

This post outlines the numerous reasons why your furnace is making a flapping sound. You’ll learn how to effectively diagnose the sound source and the steps you need to take to fix each issue. 

Something Is Stuck in the Vents

Listen closely to the flapping noise coming from the vents. Try to identify what it sounds like exactly. There’s a distinct difference in the flapping sound a piece of paper makes compared to a large sheet of metal. If it sounds like a piece of plastic or paper flapping, it could signify that something is stuck in the vents. 

This is more common than you might think. When something enters the vents, such as a wrapper or a piece of paper, it might be large enough to get stuck inside the ductwork. Air will continue to flow naturally as the piece of paper isn’t strong or large enough to cause a blockage. And as the air flows through, the paper will continuously flap, creating an unpleasant sound.

How To Fix

When a piece of paper or plastic wrapper gets stuck in the ductwork, you should start at the vents and work your way through the system. You can pinpoint the location of the flapping sound by going through the entire ductwork.

Here’s what you do once you find the area where the sound originates:

  1. Turn off your furnace. 
  2. Unhook this part of the vent to gain access to the interior. 
  3. Look inside to see where the piece of paper is stuck and attempt to remove it. 
  4. After removing the paper, make sure you fit the section properly back into place. You don’t want air to leak after removing the paper as this can cause new problems and sounds. 

A Part of the Ductwork Is Loose

The problem doesn’t always lie with a piece of paper that entered the vents. Sometimes, it’s the vents or the ductwork causing the flapping noise. This is why it’s important to check the entire line running through your home to understand where the sound is coming from. 

Maybe loose ductwork panels are the problem. You can sometimes even see the panels moving as air flows through the vents. The flapping sound won’t be the same as that of a piece of paper. It’ll be much louder since ducts are generally made from metal. 

How To Fix

As with the previous fix, you want to inspect the entire ductwork system to determine where the sound originates. Finding the specific point and working from there can make this process much easier. Make sure you know exactly where the ductwork runs and that you walk along all of these areas. 

When you find the source, take a look at the ductwork in this specific location. See if you can find a loose panel. A good idea would be to use a wrench or screwdriver to check for loose bolts and screws. Even when a panel seems to be fastened properly, the screws can still be somewhat loose. Fasten all the screws and bolts, then see if the sound has disappeared. 

Panels on the Furnace Compartment Are Loose

The furnace compartment handles a lot of air as it heats your home. The induction motor pulls in air from the outside. The air then travels through the burners and heats up to your desired temperature. After that, it’s blown through the ductwork. 

When there’s a loose panel on the furnace, you’ll often hear a flapping noise caused by air pushing against it. The noise is quite distinct — you can tell that it’s a large metal panel shaking and not a piece of paper.

How To Fix

This fix should not take more than a few minutes both to diagnose and correct the problem:

  1. Shut off your furnace. 
  2. Take a look at all panels on the furnace. You should also consider other parts of the HVAC, especially if the sound doesn’t seem to be coming from the furnace. 
  3. Gently push on each panel of the furnace’s housing. See if any of these panels move when you push them. If it does, that’s a loose panel. 
  4. Use an appropriate tool to fasten all loose panels you can find.

Internal Parts Are Loose

Sometimes, the problem lies inside the furnace, not the outside. You should still check the first 3 potential scenarios described above before moving on to this one. That’s because, in this case, you need to open up the furnace to take a look inside.

There are several components inside the furnace that could’ve come loose after years of use. Some of these components may create what you’d describe as a flapping noise when they’re loose. The heat exchanger is a good example. This part acts as a shield to keep combustion gases from entering the ductwork. It’s a relatively thin sheet of metal that can cause a metal flapping noise when loose.

How To Fix

Since you need to work inside the furnace for this one, turn it off first. Consider hiring a certified professional to work on your HVAC if you’re not familiar with electronics. 

Follow these steps to fix a loose internal component:

  1. Switch the furnace off.
  2. Remove the main panel using an appropriate tool.
  3. Look inside the furnace. Ensure every part is located where it should be and that nothing is hanging out of place. 
  4. Take a look at the heat exchanger. Make sure it is fastened properly at all corners. 
  5. Check the fans inside the furnace. Make sure there’s nothing stuck to them. A piece of paper or plastic stuck in the fan is another potential cause of the flapping noise.

Summary

A flapping noise coming from your furnace often indicates that something is loose. Air is continuously pushed through the HVAC system, meaning loose panels or other parts can cause a consistent flapping noise as the air moves. Sometimes, an internal component is loose and flaps around.