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Why Is My Furnace Making a Loud Noise? A Phoenix Tech Answers

December 18, 2019

If your furnace is making a loud noise, you shouldn’t ignore it. Noises coming from your furnace usually indicate that something is loose, broken or detached. These issues can turn into a more expensive repair (and a potential safety issue) if it’s not addressed in a timely manner.

You shouldn’t ignore your furnace if you hear noises like:

  • Banging
  • Rattling
  • Scraping

Below, we’ll dive into what can cause each of these noises and more importantly, what you should do if you hear them.

Don’t trust your own diagnosis and rather have a professional take a look at your furnace? We totally understand. We offer flexible scheduling so that we can come take a look (and hopefully repair your furnace) at a time that works for you. Learn more about the furnace repair services we offer or…

Loud Noise #1: Banging

If you hear a loud banging coming from your furnace, it’s likely because of:

  • Delayed ignition
  • Expanding/retracting air ducts

Delayed ignition:

If you hear a loud banging noise right when your furnace turns on, the most likely reason is that there’s a delayed ignition.

Let’s break down what’s happening when there’s a delayed ignition:

  1. Your thermostat sends a call for heat to your furnace and the gas supply to your furnace turns on.
  2. Your furnace fails to light that gas immediately.
  3. Gas continues to flow to your furnace and when your furnace eventually lights, a small explosion occurs due to the buildup of gas (which sounds like a bang).

Delayed ignition can be caused by a few things, including:

  1. Uneven gas/air mixture- Your furnace needs the right mixture of gas and air to light. If there is too much or too little air/gas, your furnace will struggle to ignite, which can delay your furnace from lighting.
  2. Restricted, dirty or clogged pilot light- Your pilot light is what signals your burners to turn on. However, if your pilot light is restricted, dirty or clogged, it will struggle to stay lit (or the flame may not be strong enough), which can result in a delayed ignition.

If you think this noise is coming from your furnace itself, you’ll want to contact a technician as soon as possible to check it out. It’s not safe to allow gas to build-up in your system, so this isn’t a repair you’ll want to sit on.

Expanding/retracting air ducts:

If you hear a banging noise a few feet away from your furnace and a few minutes after your furnace turns on, it’s likely being caused by expanding or retracting air ducts.

When your furnace blows air into your ducts, the pressure inside the ducts can sometimes push them outward causing a banging or popping noise. On the other end of your system (the “return side”, i.e. the side that pulls air into the system), there can be too much negative pressure, which can also cause a bang or popping noise.

Now, if your duct system is healthy, you shouldn’t be hearing a banging noise.

Air ducts that expand/contract are usually the result of:

  • Undersized ductwork
  • A dirty air filter
  • Closed air vents or dampers
  • Dirty ductwork

What to do: If you think the noise is coming from your ducts, you should check your air filter and replace it if it’s dirty.

Dirty air filter

A clean filter (left) next to a dirty filter (right)

If your furnace is still making a loud noise an hour or so after you’ve replaced your air filter, it’s time to call a technician to come and take a look. They will be able to let you know exactly what’s causing your ducts to expand/retract and what should be done to fix it.

Loud Noise #2: Rattling

If you hear a rattling noise coming from your furnace, the most likely reasons include:

  • A loose panel
  • A leak or crack in your heat exchanger

Loose panel:

If the screws holding the panel on the front of your furnace are loose, it can make a rattling noise when your furnace turns on.

Leak or crack in your heat exchanger:

Your heat exchanger is the part of your furnace that actually heats your home’s air.

parts of a gas furnace

If there are cracks in the exchanger, it can make a rattling or cracking noise when it first turns on. Your heat exchanger is made of metal, so the rattling noise you’re hearing is the sound of the crack in the heat exchanger expanding as it begins to heat up.

Cracked heat exchangers are pretty rare, so it’s unlikely that this is your issue. However, if you think there’s any possibility that this is your issue, you want to call a technician ASAP.

A cracked heat exchanger can release the gases produced during combustion into your home. Some of these gases, like carbon monoxide, are poisonous, so you’ll want to handle this as soon as you can.

Loud Noise #3: Scraping

If you hear a scraping noise that sounds like metal against metal, it typically means that a part of your system is loose.

Most likely, the blower fan has come loose and is hitting the casing around it.

Our recommendation would be to call a technician as soon as possible. If you just started hearing this noise, a tech may be able to simply tighten the blower fan. However, if the blower fan has been detached for a while, it could be broken and may need to be replaced.

Still dealing with a loud furnace? Call Phoenix’s most-trusted pros: Patrick Riley

If your furnace is still making loud noises, you should reach out to a trusted technician for help. The longer you ignore furnace repairs, the more likely they can turn into expensive repairs or replacements.