6 Common Noises Your Furnace Makes (and Why They Happen)

Scott Dylan Westerlund
Updated October 28, 2021
Woman and cat sitting with a blanket
Alena Ozerova - stock.adobe.com

If your furnace is on the fritz, you'll likely hear some strange sounds

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If you hear rumblings coming from your furnace, you’re probably wondering what it means. While there are many reasons why your furnace might be making weird noises, it’s best to call a professional to ensure it’s still safe to use. 

Take a look at six common furnace noises, and if you can DIY the fix or need to call in a furnace repair person.

1. Bangs and Booms at the Start of the Cycle

Are you hearing a banging or booming when your furnace burner cycles on? Learn the cause (and what you can do about it), so you can get back to binging Netflix in peace.

Why It's Happening

Delayed ignition. When everything is in working order, your pilot flame or electric ignitor will ignite gas flow from the burner the instant your thermostat opens the gas valve. Even the slightest delay causes a puff of unburned flammable gas to stay in your furnace's combustion chamber.

When that cloud lights up behind schedule, loud sounds radiate throughout your metal furnace. The sound is amplified as it bumps along through your ductwork.

You now know why it sounds like there's a rogue drummer running loose in your ductwork. However, you may be wondering what's causing the delayed ignition in the first place. 

Here are the common reasons why furnace ignitions get delayed:

  • Defective ignitor

  • Weak flame on pilot light

  • Burner malfunction

  • Low gas pressure

The Fix

You'll need to first narrow down why your ignitor is delayed before correcting the problem. Going through the checklist may require shutting down the furnace wall control to safely access wires and valves in the control panel. Get an expert to help you get a quick answer safely.

The good news is that the same problem that's causing the banging and booming may actually be making your furnaces less efficient. You might have a lower energy bill to look forward to after the right part is cleaned, readjusted, or replaced during a furnace repair call.

2. Intermittent Banging and Booming

You’ve just fallen asleep when a loud banging sound wakes you from your slumber. You drift back into dreamland a bit later, only to be awoken again. What is the cause of these awful intermittent furnace noises?

Why It's Happening

Blame this one on expanding and contracting ductwork. It happens because of positive and negative pressure caused by heated and cooled air being pulled through your ducts.

The Fix

While intermittent banging and booming won't necessarily cause problems, there are fixes if the sounds are disruptive. The first is to fortify the flimsy sheet metal used for most ductwork with heavy-gauge ductwork that has more stability in extreme temperatures. The second is to have round ductwork installed in place of hard edges for smoother expansion and contraction.

3. Dry Squealing

Ranking high on the list of irritating sounds is a furnace that makes a dry, grating squealing sound. You can put on noise-canceling headphones to block it out—or better yet, fix the issue and make it stop.

Why It's Happening

This is a telltale sign that your belts or bearings are worn. The easy fix is lubrication. However, badly worn belts and bearings need replacement.

The Fix

You can breathe a sigh of relief because this is a relatively easy, inexpensive fix. However, you'll still want to call in a furnace pro to do it right because your furnace should get an anti-grime cleaning, tune-up, and inspection if your belts or bearings are worn enough to need attention.

4. High-Pitched Squealing

Do you have mice in your walls or a banshee in your backyard? Nope, that’s just your furnace making a horrible high-pitched, whistle-like squeal. But don’t worry, there’s an easy solution.

Why It's Happening

Unlike a dry squeal that just means a bearing or belt is worn, high-pitched squealing usually means an airflow issue. What you're hearing is your furnace's blower struggling to take in enough air.

Here's why your blower might be struggling with air intake:

The Fix

Check that vents are open without obstruction. Next, replace your air filter.

5. Rattling

Man checking inside furnace parts
Tomasz Zajda - stock.adobe.com

Is the ghost of Jacob Marley rattling his chains around your home? Unfortunately not. That irritating noise is actually coming from your furnace, not a specter sent to teach you a life lesson.

Why It's Happening

Rattling is usually caused by one of two issues. The first is that a furnace part is broken or loose. This usually means a panel, ductwork connection, or fan belt.

The second common cause is a cracked heat exchanger. Pivot to this theory if you seem to hear rattling right after your furnace turns on. What you're likely hearing is the sound of metal coils expanding.

The Fix

Have the usual suspects for rattling sounds inspected with a careful eye on the heat exchanger. If it turns out that your heat exchanger is broken, replacing your furnace might be the best option if it's older.

6. Humming

Last but certainly not least on our list of problematic furnace noises is humming. If you’re hearing a constant humming or low-frequency buzzing, you can be semi-certain your home is not under attack by rabid hummingbirds. However, it may mean your furnace is failing, and you should have it looked at ASAP.

Why It's Happening

While buzzing is one of the more low-key furnace sounds, it often ends up being the most serious. Humming often means a major electrical component of your furnace is failing. 

This includes:

  • Blower motor

  • Blower motor capacitor

  • Inducer motor

  • Transformer

The Fix

Consider this an electrical problem that requires caution. Bring in a furnace pro to carefully look for faulty or aging electrical furnace components.

When to Call a Furnace Professional

Abnormal sounds mean your furnace is struggling. Every day you wait for an inspection can cause more stress on your furnace's components. Think of furnace noises as an SOS. Help your furnace out (and keep you and your family safely heated) by DIYing what you can and hiring an expert for the rest. Get your furnace noise taken care of by finding a local furnace repair pro today.

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