Why Does My Furnace Make A Knocking Sound [ SOLVED ]


Rima Chatterjee

Is your furnace making a knocking sound? Cold weather can make it work harder, leading to noise. Parts becoming unbalanced can create banging noises.

Maintenance and inspections with an HVAC technician can help. They can replace worn fan belts or bearings. Dirty burners and air filters can also cause loud noises.

Don’t ignore these signs. Schedule maintenance with an expert today. This will help make sure your furnace runs smoothly and efficiently this winter. 

Common Causes of Knocking Sound in Furnace


To identify and fix the source of a knocking sound in your furnace, I will provide you with some common causes as well as their solutions. In this section, we will discuss the following subsections briefly. Knowing the potential causes and remedies of these issues can help you address the situation before it worsens and causes harm to your heating system.

Unbalanced Blower Wheel

A knocking sound from the furnace? It could be an unbalanced blower wheel. This wheel is vital for distributing hot air around the house. When it gets dirt build-up or wears, it can make a knocking noise.

CauseSolution
Dirt Build-UpClean the blower wheel with a soft brush and vacuum. Get annual maintenance from a pro.
Wear-and-TearReplace the blower wheel with a new one.

An unbalanced wheel can also affect furnace performance and energy bills. Don’t wait for things to get worse. Get maintenance from a professional to keep your heating system running smoothly. 

Dirty Burners

A dirty burner can cause a knackered furnace. An air filter that’s not changed regularly or a dusty furnace room can fill burners with dirt and debris. This blocks or restricts airflow and causes clogged burners resulting in a loud knocking sound when your furnace starts up. This can lead to permanent damage, requiring expensive repairs.

Clean the burners to stop this ruckus. 

  • Turn off the furnace, 
  • Remove the access panel
  • clean the burners with a soft brush or vacuum.

5-15% energy savings come with replacing a dirty filter. This maintains your furnace and saves you money on energy bills. 

Broken Fan Belt

A furnace is a must-have home appliance and needs regular upkeep. A common source of knocking sounds in furnaces is a broken fan belt. This can be bad news, so it’s important to know what to do.

  • The broken belt means insufficient air flow, and too little heat getting around the house.
  • The furnace starts banging, as there’s no motor torque coming from the belt.
  • Air conditioning will also not work, due to the same motor running on the fan belt.
  • Worst of all, a broken fan belt can cause overheating and more costly repairs or replacement.
Why Does My Furnace Make A Knocking Sound

Broken furnace belt causing noise

If you think your furnace’s knocking is from a busted fan belt, get an HVAC pro right away. Postponing repairs can cause more harm and cost more.

Loose or Worn Bearings

Does your furnace sound like it’s knocking? It could be a sign of worn or loose bearings. Here’s what you need to know:

  • If the bearings are not working properly, they can become loose over time.
  • This knocking usually comes from the blower fan or motor, which relies on the bearings to turn.
  • If left unchecked, these loose or worn bearings could ruin your furnace.

Call a professional to check and fix the problem right away. Plus, regular maintenance by a reliable HVAC contractor can help stop issues like these before they get too bad. 

Poor Installation

A bad installation of your furnace may cause many issues, like a knocking sound. This is usually due to incorrect parts or their wrong placement.

Therefore, it’s essential to hire an educated technician. They’ll make sure all parts are correctly assembled and placed, decreasing the chance of breakdowns and noises.

In addition to experienced help, you must frequently check your furnace. Inspect all components to keep it in good condition and stop any potential noise.

Why Does My Furnace Make A Knocking Sound

Improper installation and dust causing noise in furnace

Dirty Air Filter or Furnace Filter

When it comes to the knocking sound in your furnace, a dirty air filter or furnace filter could be the cause. Clogged filters prevent proper airflow, making the furnace work harder and resulting in that noise. Here’s the lowdown:

  • A dirty filter reduces your HVAC system’s efficiency, so it has to work harder. This extra strain can create a knocking sound you don’t want to ignore.
  • Make sure to replace or clean your filter at least every three months. If it’s a reusable one, clean it following the manufacturer’s instructions.

Other issues could cause the same sound too, such as loose parts or worn-out components. If left unaddressed, small problems can lead to costly repairs or replacement. 

Issues with Gas Valve

The gas valve is an essential part of your furnace system. It could be the source of a knocking sound if clogged with debris or stuck in an open/closed position.

It’s important to address any gas valve issues ASAP. Malfunctioning valves can cause excess heat, higher energy bills and safety hazards. Plus, they can cause discomfort during colder months.

Regular maintenance checks are key to preventing gas valve problems. This will ensure your furnace is running efficiently and safely. Don’t let a faulty gas valve risk your home’s comfort and safety. Contact a professional HVAC technician right away if you suspect a problem with the gas valve.

Hair in Ductwork

Knocking sounds from your furnace can be caused by a few things. Pet hair in the ductwork is a common culprit. As our furry friends shed, their hair can block the system over time. This sound is a sign that air pressure is building up.

Dirty air filters only make it worse, congesting the ductwork even more. To avoid this, replace the HVAC filter regularly and brush or vacuum pet coats often.

Clean ducts benefit both your air quality and the efficiency of the furnace. Don’t ignore any knocking sounds. Address them right away to prevent further damage.

Signs of Knocking Noises in Furnaces


To help you identify the cause behind the knocking noise in your furnace, we will guide you through the Signs of Knocking Noises in Furnaces with Loud Noises, Banging Sounds, Squealing, and Humming being the sub-sections. Detailing each sub-section and their causes can provide you the solution for the issue you are experiencing with your furnace. Keep reading to become familiar with the reasons and fixes for these noises.

Loud Noises

Loud noises from a furnace often signal that something is wrong. Grinding, screeching, and banging sounds can indicate different issues, such as loose parts, airflow or filter problems. If components are not maintained, it can also lead to loud noises. Worn belts and bearings, damaged pilot mechanisms, dirty burners and vents, and a blocked air filter can cause noisy operations.

In addition, strange sounds from furnaces may be a sign of carbon monoxide leaks. These leaks can cause severe health risks if not addressed quickly. According to David Kao (Forbes, 2020), regular maintenance by an expert is necessary in order to identify small issues before they cause expensive damages. 

Banging Sounds

Banging noises coming from your furnace can mean several things. The most common culprits are loose ductwork or a faulty blower wheel. Blocked burners can also be the source of loud banging. If left unattended, these issues can lead to costly repairs. Or worse, accidents.

  • Other causes of banging noises can be delayed furnace ignition or dirty burners. 
  • Overheating burners can cause ignition delay, leading to small explosions. 
  • Dirt buildup can make combustion inefficient, resulting in explosive fuel buildup and banging sounds.

Squealing

A high-pitched squeal? Could be a serious problem with your furnace. It could be a defective blower motor, belt or air filter. Don’t wait. Call a professional. They’ll check for issues and solve them fast.

Squealing can also be caused by rattling pipes in the heating system. Vibrations from misaligned ducts make irritating noises. Maintain your furnace and ductwork to avoid further problems.

Humming

Unattended humming will cause serious damage. Replace old furnace parts with new ones for a sound-free life. 

  • Clean or replace air filters every 1-3 months
  • Dirty filters can cause vibrations leading to humming. 
  • Maintenance of the furnace is crucial
  • A pro technician should check the system annually

How to Fix Knocking Noises in Furnace?


To get rid of those annoying knocking noises in your furnace that are keeping you up at night and preventing you from enjoying the warmth of your home during the cold weather, follow these tips with sub-sections as solutions. Regular maintenance, inspecting components, cleaning blower wheel and burners, replacing air filter or furnace filter, scheduling expert technicians to look at the issue, changing belts and bearings, and balancing the blower wheel are all key ways to address the issue and get your heating system back to its optimal function.

Regular Maintenance

  • Change the air filter every 1-3 months for peak performance. 
  • Clean the burner ports and blower fan with a brush, vacuum, or air compressor. 
  • Check ductwork for damage or leaks. Watch out for faulty wiring, damaged parts, or rust. 

Schedule an annual tune-up with a pro HVAC technician. They’ll inspect electrical connections, blower motor, condensate drain lines, and burner ignition. Keep filters clean and surroundings dust-free to prevent knocking noises and steady heating. 

Inspect Components

To banish those knocking noises from your furnace, take a look at its components. Let’s break it down to figure out what steps you need to take. Check out the table below for more details.

ComponentsPossible Issues
Furnace FiltersLook for any clogs or dirt that can block the airflow and mess with the blower motor.
MotorsCheck if the bearings are worn-out or the belts are loose. This can cause vibrations or rattling.
Burner AssemblySee if there are any cracks, rust, or debris that can cause popping sounds during ignition.
DuctworkMake sure none of the ducts are damaged, blocked, bent, or loose to prevent air leakage and whistling.

Inspecting the components correctly will help you identify any problems. Be aware of any sparks or gas smells while examining. Don’t let the knocking noise take over your home. Check your furnace now and save money on potential repairs or replacements. 

Clean Blower Wheel and Burners

To fix knocking noises in your furnace, clean the blower wheel and burners! Follow these 3 easy steps:

  1. Turn off the furnace and disconnect it from the power source to avoid accidents.
  2. Locate the blower compartment, take out the wheel, and clean it with a brush and vacuum.
  3. Remove the burner assembly and clean its parts with a wire brush. Inspect for wear and tear before reassembling.

Cleaning the blower wheel and burners should be done every 6 months. If you’re not sure how to do it, call a technician.

To prevent knocking noises, remember to:

  • Replace dirty air filters regularly.
  • Install an air purifier or humidifier.
  • Get an annual maintenance by a professional technician.

By following these tips and cleaning your blower wheel & burners, you can keep those pesky knockin’ noises away and make sure your furnace runs smoothly. 

Replace Air Filter or Furnace Filter

To fix the knocking noises in your furnace, you must replace the air filter or furnace filter. Dirt and debris build-up in the filter restricts air flow and causes pressure on the motor. Here’s what you need to do:

  1. Turn off your furnace.
  2. Note the size of the current filter.
  3. Locate the filter (could be on the blower door or louvered door compartments).
  4. Carefully remove the old filter.
  5. Insert the new filter properly.
  6. Turn your furnace back on.

It’s best to replace filters every 3 months. Otherwise, it can cost you money and damage your furnace. 

Schedule Expert Technicians to Look at the Issue

It’s essential to book expert technicians when your furnace makes knocking noises. Calling in the pros can save you both time and money in the end. Here’s a guide on how to do it:

  1. Look up HVAC companies near you
  2. Inquire about their qualifications and experience with furnace repairs
  3. Ask availability for an appointment
  4. Let them know the make, model, age, and type of noise heard
  5. Agree on a date and time for inspection
  6. Prepare relevant information or paperwork

Compare pricing from different companies before you decide. Also, ask follow-up questions if something is unclear. Don’t forget to set up regular maintenance appointments. Preventing future problems is just as important as fixing the current ones.

Change Belts and Bearings

Replace those belts and bearings, a task essential for knocking noises in your furnace. Neglect it and you’ll be facing a complete breakdown of the unit with higher repair costs. Here’s a 4-step guide:

  1. Disconnect the power and locate the access panel.
  2. Remove the access panel and check the belts. If cracked or worn out, replace them.
  3. Loosen the tensioner pulley screw with a wrench, remove the old belt, place the new one around the pulleys, tighten the screw and adjust.
  4. Check for wear or tear on shaft bearings or motor fans. Replace if damaged.

When replacing belts, get high-quality ones designed for your furnace model. Keep safety in mind when dealing with electrical devices.

Failing bearing can cause slow system performance, improper airflow and higher electricity bills. A homeowner who didn’t change his bearing saw his motor destroyed, costing him $1000 instead of $50 for changing the bearing beforehand. Don’t neglect small problems; they become bigger expenses if ignored.

Balance the Blower Wheel

Do you hear loud banging coming from your furnace? It’s time to balance the blower wheel! This will make sure all parts work together nicely, so no more noise. Here are 3 steps to do it:

  1. First, turn off the furnace’s power.
  2. Next, open the access panel to get to the blower motor.
  3. Lastly, use a balancing clip or weight kit to make sure the blower wheel stays even.

And don’t forget to check the bearings and lubricate them if needed. Taking good care of your furnace will help it run more smoothly and last longer.

Conclusion


Do you hear a loud knocking sound coming from your furnace? It could be due to issues with components like the blower wheel or fan belt. It could also be from poor installation or dirty burners. To stop harm and keep heat up, pay attention to sounds your heating system makes. Don’t ignore them. If the issue continues, you may need to replace components or get a new furnace.

About the author

Debarghya Roy: A heating systems author, Passionate about energy efficiency and sustainability, Sharing insights and empowering readers through informative blog articles.