How to Diagnose a Noisy Furnace

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Overview
  • Total Time: 1 - 2 hrs
  • Skill Level: Intermediate
  • Estimated Cost: $10 to $30

A noisy forced-air furnace/AC system is not only annoying, but it can also be a sign of a serious problem. If your HVAC system begins to make sounds you've not heard before, don't ignore it. While some problems causing noisiness are relatively innocent and can be corrected easily, other furnace sounds can lead to major, expensive repairs if you don't address the underlying problem quickly.

These sounds don't appear only during the heating season. Forced-air AC systems use the same components, so if you hear strange sounds during the summer cooling season, also pay attention.

Here’s a process for diagnosing possible problems indicated by a noisy forced-air furnace system.

What You'll Need

Equipment / Tools

  • Screwdrivers
  • Nut drivers
  • Plumber's drain snake (if needed)
  • Shop vacuum
  • Allen wrench (if needed)

Materials

  • New furnace filter
  • Motor lubrication oil (as needed)

Instructions

It’s a good idea to call a professional if you’re experiencing serious problems with your furnace. Tackling most furnace repairs yourself can damage your unit and lead to home-threatening problems such as gas leaks. Fortunately, not all furnace fixes require the work of a professional. You can easily replace a filter, check your flue, and oil your blower motor.

Troubleshooting a noisy furnace involves going through a process of addressing the most likely easy fixes before dealing with the complicated issues that may require a furnace technician. There will be no need to complete all steps if you arrive at a solution that relieves the noise in your furnace.

  1. Listen to the Furnace

    When your furnace begins to make uncharacteristic noises, listening carefully may help you identify the problem. This may steer you to the solutions you can try yourself, and it will also help you describe the problem accurately if you need to call a service technician.

    • Scraping: A scraping or metal-against-metal sound most often is caused by a loose or broken blower wheel, or a broken motor mount. Blower wheels can come loose and hit the blower housing, causing a scraping sound. If left unattended long enough, a loose blower wheel can damage itself and the housing. A broken blower wheel will vibrate in its housing, resulting in squealing, scratching, and banging sounds. A broken motor mount causes the blower wheel to drop and rattle against the housing. Shut your furnace off if you have a compromised motor mount. A distinct scraping sound is usually a sign you should call a furnace technician for repairs.
    • Thumping: Thumping and vibrating sounds are the results of an unbalanced blower wheel. The motor itself can also become out of balance. Have a pro take a look to avoid further damage to your furnace.
    • Humming: If you hear humming but your furnace is functioning correctly, you likely have a loud transformer. Furnace problems and humming are sometimes related to a bad interior fan or capacitor.
    • Squealing: A lack of lubricant or a loose fan belt can cause high-pitched squealing. Applying oil or tightening your blower belt should stop the noise. Call a pro if you’re unsure how to perform either of these jobs. Your furnace's owner manual may give instructions on how to do this; there are also on-line articles and videos that will describe the process for your furnace model.
    • Banging: The start-stop cycle of your furnace fan can cause the sheet metal siding to bend inward, resulting in a banging or popping sound. This could be the sign of an undersized duct, closed vents, or a clogged filter.
    • Rumbling: A low rumbling can indicate a problem with your burner. Turn off your furnace and call a professional if you notice a constant rumbling.

    Turn off the power and gas to your furnace if you notice loud or unusual sounds. Continuing to use a malfunctioning furnace can cause serious damage to the unit and require expensive repairs.

  2. Replace the Filter

    While a dirty air filter can cause many furnace problems, it can can also be the source of a noisy furnace when it causes an air pressure build-up in the furnace.

    Furnace filters should ideally be replaced every month, and if you haven't replaced yours recently, start by doing so. Filters vary in terms of lifespan and effectiveness, so read the technical requirements for your filters and your furnace.

    Replacing a filter takes only seconds, but it is a good idea to shut off the furnace as you remove the old one and insert the new filter. Make sure the filter is oriented correctly in the filter slot, which is usually located in the air return duct or the entrance to the blower chamber. Arrows on the filter indicate which way it should be positioned in relation to the air flow through the system. After inserting the new filter, replace any access panels you removed, and turn the furnace back on.

    With a new filter improving the air flow through your furnace, you may now find that it runs more quietly.

    Health Tip

    Replacing your filter is a good time to consider installing a high-end filter that can filter out fine pollens or even virus particles. While they are expensive, these high-end filters have demonstrated health benefits when changed monthly.


    The filtering ability of a filter is identified by its MERV rating. Medium-quality filters usually have a MERV rating of 8 to 10 while those with electrostatic function capable of capturing bacteria and virus particles have a MERV rating of 13 to 15.

  3. Clean the Furnace and Floor Vents

    While it usually doesn't make a huge difference in noisiness, filter replacement is a good time to vacuum out the floor of the furnace. Make sure the furnace is shut off, then use a shop vac to remove dust and debris from the floor of the furnace and filter chamber.

    You can also remove floor grates for the furnace ducts throughout the house and vacuum out the ducts at this time.

  4. Inspect the Flue/ Vent Pipes

    If the flue or vent pipes that exhaust combustion gases from the furnace have any obstructions, it can cause a pressure build-up in the furnace that might cause it to run noisily. Normally, the symptom of this kind of problem is a furnace that stops cycling on and off correctly, not one that runs noisily. But partial blockages can potentially create slight pressure increases that cause unusual noises in the furnace.

    Make sure the furnace is shut off before inspecting the flue or vent pipes. Normally this is done from the exit point for the flue or vent pipe. This is usually on the roof for a standard furnace with a flue/chimney, or from the side of the house with a high-efficiency furnace with a PVC venting system.

  5. Oil the Blower Mower

    A lack of lubrication in the blower motor can diminish your furnace’s effectiveness and cause unwanted noise. Adding oil to your furnace requires some mechanical know-how. An experienced DIYer can certainly do it, but you should call a pro if you’re unsure about tackling this job.

    Start by turning off the power to the furnace. Cut the power to your furnace by turning off the circuit breaker at the main electrical panel. Then, locate your access panel and loosen the screws to remove it.

    The blower motor housing is located near the base of the furnace. Remove the bolts holding the housing in place, and carefully extract it from the furnace. Be sure not to stretch the motor’s wires. Rest the motor and housing just outside of the furnace.

    Remove the set screw on the side of the housing with an Allen wrench, then remove the motor from the housing. The oil ports on the motor are usually marked with plastic or metal plugs. Remove the plugs and squeeze two to three drops into the ports.

    Reattach the motor to the housing, then place the housing back into the furnace and secure it in place. Replace the access panel, turn the furnace back on, and test the operation, listening to see if the noisiness has been cured.

  6. When to Call a Furnace Technician

    If these steps fail to quiet a noisy furnace, it's likely you are facing a bad blower motor or some other problem that will need the services of a qualified furnace technician. Costs for a furnace blower replacement can cost $500 or more unless you have some type of service contract or appliance insurance, so before investing this, consider the age and energy-efficiency of your present system. With an old, standard-efficiency furnace, it may make more sense to replace the entire furnace with a new high-efficiency model.