Why Is the Furnace Making a Loud Humming Noise?

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You should have a loud humming noise from your furnace checked out by an HVAC pro.
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Are you suddenly surprised by your furnace making a loud humming noise? Hearing any new noise from your furnace can be alarming, and it's something you should have checked out by an HVAC professional who can do a system checkup and find the source of the noise. You can troubleshoot your HVAC system based on the type of humming sound, but most causes of humming noises require repair from an HVAC professional.

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A loud humming noise coming from a furnace may indicate a problem with the blower motor, transformer or capacitor. Loose parts or loose ductwork could also be the issue.

Blower Motor Problems

If the humming sound comes from the blower area and is sudden and loud, the source is often the blower motor. Sometimes, lubricating the blower motor will stop the humming sound. Shut off the system and let it cool before adding lubrication. Turn the furnace back on to see if the humming is gone.

If you still hear humming coming from the blower, it could be an indicator that the blower motor is failing and will need to be replaced soon. This is often a job for a professional HVAC technician. Blower motor problems can start with poor care, such as not changing your air filter regularly. When the filter is clogged, the motor has to work harder, which can put extra strain on it.

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Issues With the Transformer

Your HVAC unit has a transformer inside that can have issues that result in a humming sound. If your home's electrical current passes through the transformer, you might hear a humming sound. This can be a sign that the transformer will need to be replaced soon. If you suspect the transformer, have an HVAC technician confirm your suspicion. Having it replaced before it fails completely is ideal since your HVAC system will stop running if it completely fails.

Vibrations in Ductwork

Ductwork is a key part of your HVAC system, and it carries the cooled or heated air. The thin metal sheets that make up the ductwork can become more flexible over time, and the seams sometimes become loose. When this happens, the ductwork can create a humming sound when the heated or cooled air passes over the metal.

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If the issue is due to loose seams, you can sometimes tighten the screws with a screwdriver to tightly connect the pieces of metal. This is something you can do on your own. If the ductwork is in poor condition, it might need to be completely replaced by an HVAC technician.

Loose Internal Parts

If you haven't had an HVAC tune-up in years, the humming sound you hear could be due to loose parts inside your system. Nuts, bolts and belts are just a few of the internal components that can come loose and cause vibrations that create a humming noise. It might not seem like a big deal, but those loose components can eventually cause serious damage to your furnace.

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Since the mechanical components are intricate and must be precise, it's best to schedule an inspection with an HVAC technician to determine if this is the cause. The technician can tighten any loose parts and replace worn parts that are contributing to the humming sound. Your HVAC professional might notice other issues as well that can be fixed now before they get worse and more expensive.

Broken Furnace Capacitor

A furnace capacitor is another crucial component that can cause humming sounds if it quits working. This is often the problem if you have your HVAC unit turned on, and you can hear the humming noise, but the motor isn't working. Have a licensed HVAC professional install a new capacitor to get the unit running again.

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Shelley Frost combines her love of DIY and writing in her freelance career. She has first-hand experience with tiling, painting, refinishing hardwood floors, installing lighting, roofing and many other home improvement projects. She keeps her DIY skills fresh with regular projects around the house and extensive writing work on the topic.