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Why Is My Furnace Making a Loud Noise? A Michigan Tech Explains

Worried about that loud, obnoxious noise coming from your furnace?

Don’t worry, we’ll help you troubleshoot the noise you’re hearing.

We’ll tell you right off the bat that loud noises are not normal and are usually a sign of a problem that a professional will need to repair.

In this blog, we’ll explain what causes the following furnace noises:

  • Clicking
  • Banging
  • High-pitched squeal or whistling
  • Humming or buzzing
  • Rattling

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Noise #1: Clicking

If your furnace is making a clicking noise, it usually means there’s an issue with the ignition system.

The ignition system is responsible for creating combustion, which is how your furnace creates heat. If the furnace can’t ignite for some reason, then you may hear a clicking noise as the igniter struggles to light.

Common ignition issues that cause a clicking sound include:

  • No gas is reaching the system due to a faulty ignition board or a bad valve
  • The burners are dirty
  • The flame sensor is broken
  • The pilot light is extinguished or the thermocouple is broken (only applicable to older furnaces)

Besides the ignition issues above, a broken fan motor can also cause a clicking noise if the bearings are loose.

What to do: Have a professional inspect your furnace ignition system and fan motor.

Noise #2: Banging

Depending on when you hear the loud banging noise, you may have either:

  1. A delayed gas ignition issue
  2. Expanding/contracting air ducts

Cause #1: Delayed gas ignition issue

If you hear the banging noise when your furnace first turns on, then your furnace may not be lighting right away like it’s supposed to.

When a furnace doesn’t ignite right away, gas builds up in the combustion chamber. After the furnace finally ignites, all of the built-up gas creates a mini explosion, which can harm your furnace.

The following issues cause delayed ignition:

  • Too much air mixed with gas
  • Low gas supply
  • Broken pilot light (with older furnaces)
  • Dirty or misaligned burners

What to do: Contact a professional to inspect your furnace ignition system and repair any issues.

Cause #2: Expanding/contracting air ducts

If you hear intermittent banging sounds while your furnace is running, it could mean you have expanding or contracting ducts. (You’ll likely hear these noises throughout your house, not necessarily by the actual furnace itself.)

Supply and Return of Air Ducts in Home (with a Furnace)

The supply and return of air ducts in a home (with a furnace)

When hot air gets pushed through the supply side of your home’s ductwork, it creates positive pressure which can cause the ducts to expand. Additionally, when your furnace pulls in cold air to be heated on the return side, it creates negative pressure which can cause the ducts to contract.

Because ducts are often made of sheet metal, it’s normal for ducts to make noises from time to time as they expand and contract. However, if the banging noises are distractingly loud or frequent, it could mean there’s an issue.

Problems that cause extra loud ducts include:

  • Dirty air filter
  • Closed or obstructed vents
  • Undersized or dirty ductwork

What to do: Contact a professional to inspect your ductwork. They can see what is causing the loud banging noise problem and recommend a solution.

Noise #3: High-pitched squeal or whistling sound

A high-pitched squeal coming from your furnace is usually the result of an issue with the blower.

The blower in your furnace is powered by a motor, which has bearings that can wear out because of age or overheating. Overheating is usually caused by dirt buildup or a bad capacitor.

On the other hand, a high-pitched whistling sound is usually the result of a low airflow problem. Low airflow means that the blower is struggling to pull in enough air to be heated.

Common causes of low airflow include:

  • A dirty air filter
  • Closed or obstructed air vents
  • Dirty or damaged ducts

What to do: If you hear a high-pitched whistling sound, try changing your air filter and opening all vents to see if that fixes the problem. If that doesn’t work or you hear the squealing sound, contact a professional for further assistance.

Noise #4: Humming or buzzing

A dull hum is normal while your furnace is running. But if you hear a loud humming or buzzing sound coming from your furnace, it usually means your furnace has an electrical problem.

The most common electrical issues that cause a humming or buzzing sound include:

  • A failing blower motor capacitor
  • An aging or unlubricated blower motor
  • A faulty transformer
  • Bad inducer motor

What to do: Contact a professional to inspect the electrical components of your furnace to pinpoint the issue and fix it.

Noise #5: Rattling

A rattling noise coming from your furnace means 1 of 2 things:

  1. Your furnace has a loose part
  2. The heat exchanger is cracked

Cause #1: Your furnace has a loose part

A furnace is a complex piece of mechanical equipment, which means it has a lot of moving parts. Sometimes parts can become loose over time or because they were not tightened correctly.

Common loose parts that make a rattling sound include:

  • Bearings in the blower motor
  • Panels around the furnace
  • Connections in the ductwork
  • Worn blower motor fan belt (only applicable to older furnaces)

What to do: Contact a professional to inspect your furnace and ductwork and tighten any loose parts.

Cause #2: The heat exchanger is cracked

Do you hear a high-pitched rattling sound shortly after your furnace turns on?

If so, it could mean your heat exchanger has a leak. First, let’s explain what the heat exchanger is.

The heat exchanger is the heart of your furnace; it’s the part that actually heats your home’s air.

Furnace Heat Exchanger

The heat exchanger inside a furnace

The heat exchanger is made up of metal coils that are heated by combustion. When colder air from inside your home comes into contact with these hot coils, it turns warm. This warm air is then pushed back into your home, which is how your home stays comfortable.

That said, if the metal coils of the heat exchanger have a crack or leak, you’ll hear a rattling sound shortly after the furnace turns on. This is because the metal is expanding, which causes a rattling noise.

What to do: Contact a professional to inspect your heat exchanger for leaks. Unfortunately, if your heat exchanger is cracked, it will be a very expensive repair (almost equivalent to the cost of getting a new furnace).

For more information about how to verify you really have a leak in your heat exchanger, read our blog, “How Much Does it Cost to Replace a Furnace Heat Exchanger in Michigan?”.

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