While it’s perfectly normal to hear a low hum or a quiet rumble while the furnace is running, if the noise is distractingly loud, it usually means there’s a problem that needs to be addressed.
In this blog, we’ll explain what causes the following common furnace noises:
We’ll also tell you from the get-go that if you hear any of these noises, you’ll likely need to contact a professional for assistance. So, if you want to skip the explanations, go ahead and contact an HVAC pro right away.
Noise #1: A banging sound
A loud, banging sound is typically caused by two problems:
- Delayed gas ignition
- Expanding and contracting ducts
Delayed gas ignition
Do you hear the banging sound when your furnace first turns on?
If so, that likely means there’s an ignition problem. If the furnace fails to ignite at the right time, gas will build up inside the combustion chamber. When the flame finally ignites, it creates a mini explosion (hence the loud bang).
The following problems could be causing the delayed ignition:
- A faulty pilot light (only applicable to older furnaces)
- A problem with the electronic ignition system (new furnaces)
- Dirty or misaligned burners
Solution: Contact an HVAC professional to inspect the ignition system. A delayed ignition problem will only get worse, so you’ll want to have the problem fixed before it damages the furnace or becomes a safety hazard.
Expanding and contracting ducts
If you hear a frequent banging or popping sound while the furnace is running, it could be caused by expanding and contracting ducts.
You see, many ducts are made of sheet metal, which naturally expands and contracts due to pressure changes in the ductwork. This leads to a popping or banging noise that you hear throughout the house.
Some amount of banging or popping from the ducts is normal, but if the noises are getting louder or more frequent, it could be caused by the following issues:
- Dirty air filter
- Too many closed or obstructed vents
- Undersized ducts
Solution: First, check your air filter and change it if it’s dirty. If the banging noise persists, contact a professional to inspect your furnace and ducts to see what could be causing the problem.
Noise #2: A squealing sound
Do you hear a squealing sound, like the sound of metal grinding on metal?
If so, the sound is most likely caused by a problem with the blower. The blower is the part that’s responsible for pulling in cold air into the furnace to be heated, then pushing warm air back into your home.
The most common blower problems that lead to a squealing or grinding sound include:
- Worn blower motor bearings
- A loose or broken blower wheel
- A broken motor mount
Solution: Contact a professional to inspect the blower assembly and fix any problems.
Noise #3: A buzzing sound
Buzzing or loud humming sounds typically indicate an electrical problem somewhere in your furnace.
Examples of electrical problems that create a buzzing or humming sound include:
- Faulty electrical transformers
- Failing motors
- A broken capacitor
- A wiring issue
Solution: Contact an HVAC pro to check out your furnace and repair any electrical problems.
Noise #4: A rattling sound
A rattling noise from your furnace is usually caused by 1 of 2 problems:
- A loose component
- A cracked heat exchanger
A loose component
A furnace has a lot of small moving parts. If these parts loosen or become disconnected, the furnace can produce a rattling sound while it’s running.
Some examples of loose components that can cause the furnace to produce a rattling sound include:
- A loose panel
- A loose screw or nut
- Disconnected ducts
- Misaligned blower
Solution: Have a professional tighten or secure any loose parts in your furnace. The good news is this type of repair is usually inexpensive.
A cracked heat exchanger
In the worst case scenario, the rattling sound could be the result of a cracked heat exchanger.
The heat exchanger is a series of metal coils that lives in the heart of your furnace. When the furnace is running, the metal coils are heated in order to warm cool air that blows over them. However, the exchanger’s metal coils can form cracks if they are subject to extreme expansions and contractions (caused by overheating issues). This can result in a rattling sound shortly after the thermostat is turned on.
Solution: Contact a professional to inspect the heat exchanger. If the heat exchanger is cracked, it will need to be replaced because it poses a safety hazard. (A cracked heat exchanger can leak harmful combustion gases, such as carbon monoxide, into your living space.)
Unfortunately, replacing a heat exchanger is one of the most expensive furnace repairs. For that reason, you’ll want to be 100% sure you really need a new heat exchanger. Heat exchanger cracks will be visible to the naked eye, so the technician should point out the cracks to you so that you can confirm you need a new heat exchanger. You’ll also want to see if replacing the heat exchanger is covered under your furnace warranty, as that could reduce some or most of the replacement cost.