Why is My Furnace So Loud? A Florida Tech Answers

January 28, 2021

AC Troubleshooting

A low humming noise is what you want to hear when your furnace is turned on and while it’s running. That’s simply an indication that your furnace is doing its job.

However, if you hear any loud or abnormal noises, especially when your furnace turns on or off, there is probably an issue with your system that you’ll want to diagnose.

A few of the most common noises heard by furnace owners include:

  • Banging
  • Rattling
  • Whistling
  • Screeching

Below, we’ve outlined the causes of each of these noises, so based on the specific noise you’re hearing, you can get a better idea of what’s going on with your furnace and how to fix it. Don’t have time to diagnose your furnace yourself? No problem, that’s where we come in! Our expert HVAC techs have dealt with thousands of furnace issues. They’ll be able to assess and diagnose your issue and provide repair recommendations in no time. Learn more about the furnace repair services we offer or schedule an appointment!

Loud Noise #1: Banging

A banging noise is usually the result of:

  • A ductwork issue
  • Delayed ignition

A ductwork issue:

Sometimes a banging noise can be caused by pressure changes in your ductwork. If there is a pressure change between the return and supply ducts, the walls of the ductwork can move in and out, called oil canning. This duct movement can create a banging noise.

If your ducts were recently installed/replaced or your ducts are older, this could be your issue. You'll want to reach out to a professional who can assess the stability of your current ductwork and reinforce or replace any ductwork that may not be stable.

Delayed ignition:

If you have a gas furnace and you hear a loud bang after your furnace cycles on, it's most likely due to delayed ignition.

Delayed ignition happens when your burners are dirty. As soon as gas enters your internal unit, the burner should ignite the gas and start creating heat. However, if the burners are dirty, they won't light the gas immediately, which will allow some gas to build up before it's lit by the burner.

This buildup of gas creates a boom or bang noise when the burner finally lights the gas. If you think that delayed ignition is your problem, you’ll need to reach out to a professional for help. An expert will be able to safely disassemble your furnace, clean your burners and then reassemble your furnace to ensure everything is working safely and correctly.

Loud Noise #2: Rattling

Hear a rattling noise coming from your indoor unit? The two most common causes of furnace rattling noises include:

  • A loose screw
  • Cracked heat exchanger

Loose screw

The most common reason your furnace may be rattling is simply due to a loose screw.

If your furnace has recently been maintained or you recently worked on your furnace yourself, a panel is likely just loose and needs to be tightened.

Our suggestion would be to check all screws holding your furnace panels in place and ensure that they are tightened. If the rattling noise continues, you may have a cracked heat exchanger or an internal part of your system may be loose. If this is the case, you'll need to reach out to a professional.

Cracked heat exchanger

A cracked heat exchanger is a very rare occurrence, but if your heat exchanger is cracked, it can cause your furnace to make a rattling noise.

The heat exchanger is the part of your furnace that actually heats your home’s air. If there are small stress cracks in your exchanger due to overheating, they can end up creating a larger crack, which can make a rattling noise when your furnace is on.

If your heat exchanger is cracked, it can leak dangerous gases, such as carbon monoxide, into your home. If you think you have a cracked heat exchanger, or your carbon monoxide alarm is going off, you should leave your home immediately and reach out to a professional.

Loud Noise #3: Whistling

If you hear a whistling noise coming from your furnace, you’re likely dealing with one of two issues:

  • Leaking or misaligned ductwork
  • Clogged air filter

Leaking ducts or misaligned ductwork

If your ducts have leaks or are misaligned, they can create a whistling noise when air is blown through them. If your duct system is older or you recently had it cleaned or sealed, this is likely your issue.

If you have leaking or misaligned ducts, you'll probably also notice an uneven temperature throughout your home and an increase in your energy bills. If you think your ducts are leaking or misaligned, you’ll need to reach out to a professional to assess your ducts and either seal or replace your duct system.

Clogged air filter

Another reason you might notice a whistling noise coming from your furnace is because of a clogged air filter. Your furnace sucks in air from your home in order to heat that air and circulate it throughout your home.

All the air that's sucked into your furnace goes through your air filter. So, if your air filter is clogged, your furnace will struggle to pull in enough air, which can create a whistling noise. If it's been a while since you replace your air filter (1 month+), a clogged air filter is likely your issue. To fix this, check your air filter and replace it if it's dirty.

Loud Noise #4: Screeching

If your furnace is screeching, you’re probably dealing with a blower motor or wheel issue. Usually screeching is the result of:

  • Blower motor bearings that aren’t properly lubricated or are worn out
  • A blower wheel that’s misaligned or loose

Blower motor bearings

Blower motor bearings are an important part of your furnace, and bear friction for your motor while your blower motor spins. If the bearings are worn out or not properly lubricated, it can cause a screeching noise. If you think your blower motor bearings are worn out or need to be lubricated, you will need to reach out to a professional. Replacing the bearings requires taking apart your internal unit, which you will want a professional to do.

Misaligned or loose blower fan

Your furnace has a fan in it, called the blower fan, that circulates air throughout your home. However, if the fan has detached, is misaligned or is simply loose, it can hit against the blower casing when your furnace is on, creating a metal-on-metal or screeching noise. If you notice a screeching happening consistently while your furnace is on, this is likely your issue. You will need to reach out to a professional who can assess your furnace and repair or potentially replace the blower.

Can’t seem to quiet your furnace? Reach out to Red Cap!

If your furnace is still making loud noises after trying the DIY solutions above (or you have an issue only a pro can fix), reach out to us! Our experts have years of experience dealing with furnace issues and can promptly and accurately diagnose and repair your system.

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