Why Does My Furnace Make a Knocking Sound?

Your furnace should run without making any sustained discernible noise. If you hear knocking sounds while the system runs, you should get to the root of the problem quickly to prevent more serious damage.

As the local HVAC specialists in Boiling Springs, SC, we frequently get asked, “Why does my furnace make a knocking sound?” In this article, we’ll look at the top reasons.

Broken Blower Wheel

A furnace with a broken blower wheel will make a loud knocking sound while running.

If you hear this sound, turn off the furnace until a repair team arrives. The technicians will replace the broken blower wheel. The replacement process will take less than an hour in most cases.

Misaligned Furnace Blower Wheel

A misaligned furnace blower wheel will also produce a knocking sound when your furnace is running. If you keep running the system with the blower wheel out of balance, it will lead to more serious damage. Some of the top causes of furnace blower wheel misalignment include:

• Foreign material (including birds and rodents) lodged inside the return air duct
• Dislodged balancing weights

A Cracked Heat Exchanger

A common cause of knocking sounds while your furnace runs is a crack in your heat exchanger. Although the heat exchanger consists of metal, it can develop cracks over time.

The knocking sounds you hear could be the cracks expanding. If checks confirm that you have a cracked heat exchanger, you need to replace it as soon as possible. Ignoring a cracked heat exchanger leads to an increased risk of carbon monoxide poisoning.

Misaligned, Broken, or Worn-out Furnace Fan

Any problems with your furnace fan can cause knocking sounds while using the system.

If the fan is the source of the noise, it’s probably out of alignment. There might also be a foreign object caught between the blades. It’s not uncommon to find debris and dead critters blocking the fan’s movement.

If there’s something lodged between the fan, cleaning it out is often enough to restore the fan to optimal condition. However, if the fan is broken or too old, it’s due for replacement.

When people ask, “Why does my furnace make a knocking sound?” we always find that it’s due to one of the above reasons. In some cases, the noise isn’t quite a knocking sound. Customers also complain about popping or squeaking noises.

Causes of Other Types of Furnace Noise

Broken Blower Motor or Blower Assembly

If you hear loud banging sounds while your furnace is running, it could be due to a broken part in the motor or blower assembly. Turn off the furnace if you hear banging noises, and call in a professional for a thorough examination.

Motor Belt or Bearing Problems

Screeching and squealing sounds coming from your furnace usually signify a bad belt or problems with the motor bearings. Replacing the belt and lubricating the bearings will likely make the sounds disappear.

Although it may seem like it, changing the belt and lubricating your bearing is not a DIY project. Leave the job to furnace repair specialists.

The technicians will inspect the blower motor for more serious issues before lubricating it or changing the belt.

Air Duct Inflation 

If you hear your furnace make a banging or loud popping noise after turning it on, this is a classic sign of air duct inflation. The cold metal component in the air duct will expand as warm air passes through it. You don’t have to worry about this type of furnace noise. 

However, it’s still a good idea to call an HVAC professional to check out the noise. Dirty gas burners in the furnace can also cause a loud popping noise. Sediment collects on the burners as you use your furnace. If you don’t clear out the debris over time, the furnace will burn the sediment as it runs, leading to the popping noise you hear.

Comprehensive Furnace Repair Services Near You

If you find yourself asking, “Why does my furnace make a knocking sound?” you most likely need our furnace repair services. Our trustworthy technicians will inspect your furnace, make a detailed report of any concerns, and swing into action to correct the problem.

Call the Smooth Sailing Heating & Cooling team in Boiling Springs, SC, at 864-804-6400 to request a consultation today.

Are you considering installing an AC? We also offer exception AC installation and repair services. Learn more about getting the right size of AC for your room.