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Why Is My Furnace so Loud? How to Fix a Noisy Furnace

It feels like the summers are getting warmer, but that doesn't mean the winters are any milder than normal. If last year is any indication, we need to be ready for cold days ahead, even in some unlikely places.

That means getting our furnaces and heat pumps in prime condition, starting with a tune-up and test. But we know that minor issues may not become apparent until well into the winter season.

Even with your regular maintenance routine, you may catch yourself on the coldest night of the year wondering, "Why is my furnace so loud?" If you don't do anything, the sound may eventually go away, but you'll soon catch yourself wondering, "Why can I see my breath?"

Why Is My Furnace so Loud?

A loud furnace shouldn't go unanswered. There are plenty of moving parts, electronics, and processes involved in your heating system, and there are plenty of ways it can fail. You may hear one of several different sounds depending on the issue.

Let's discuss some of the causes of a noisy furnace and when they need attention.

Banging Sound From Ducts on Startup

A banging sound might be alarming, yet it's usually not a problem with the furnace. The most common cause is an issue with flexing ductwork, or "oil-canning".

When the blower kicks on and starts filling the duct with air, the duct expands and pops, making a thunderous booming or banging sound. As the furnace shuts off and releases air, it will make another boom as it contracts.

Flexing ducts are usually near the furnace, but they can occur anywhere in the home — you have to listen for them to locate them.

Installers usually bend the walls of the ductwork a little to keep them from flexing in and out. It's all about giving it rigidity so that it doesn't move.

There are two ways to give it that rigidity. The easy but non-recommended tactic is to take a hammer and hit the duct where it's causing an issue, giving it a bend that stops it from flexing.

A more permanent solution is to take a piece of metal (or even wood) and screw it onto the duct to give it strength. You can also insert a spacer inside the duct, but you need to recognize how it might affect the airflow.

Booming Sound From Furnace on Startup

A less common source of a booming sound is the gas ignition in the furnace. When your furnace turns on, your burners are supposed to ignite immediately when the gas is released. A booming sound occurs when an excessive amount of gas is allowed to build before the burners ignite, creating a larger-than-normal flame in the furnace.

If you suspect these minor explosions, or "roll-outs", are happening, check your furnace during startup. Leave the doors closed and check for any shaking or flames when you hear the noise. You may also notice burn marks or discoloration from the heat on the outside of the combustion chamber.

There are a few possible causes of roll-outs:

  • Dirty burners
  • Soot buildup in the flue
  • Cracked heat exchanger

As you can imagine, there are several dangers of roll-outs. It can damage heating system electronics or release carbon monoxide from a cracked heat exchanger. The biggest concern is a house fire.

Fortunately, house fires from central heating units are uncommon. Only about one in 10 heating-related fires occur from furnaces, which is primarily due to the flame roll-out switch. The safety feature is standard on most furnaces, cutting off the furnace when a roll-out happens.

Preventing Furnace Flame Roll-Out

Keeping your burners and exhaust vents clean and regular heat exchanger inspections can help prevent combustion gasses from building up. Doing a maintenance check before the season is the best way to prevent any safety issues.

Start by inspecting your heat exchanger for any cracks and keep a working carbon monoxide detector nearby at all times. We recommend having multiple carbon monoxide detectors throughout the house as an extra sure protection.

Before cleaning your burners and combustion chamber, make sure to cut off the electric and fuel supplies. To clean the combustion chamber, you can use a wire brush to scrape off any soot or debris before vacuuming it out with a shop vac.

When it's time to clean the burners, you'll need to disassemble the configuration to reach each one. Take a picture of the assembly and note the direction each piece faces to make it easy to reinstall.

Like the combustion chamber, you can clean dirty furnace burners with a small brush. Use a can of compressed air to blow out any debris stuck inside the chamber. While everything is disassembled, this is a good time to clean or replace your flame sensor.

Humming Sounds From Blower

If your thermostat setting is higher than the indoor temperature, your furnace should turn on. In some instances, you may hear the furnace try to start but there will be no air flowing from the vents.

One cause could be a bad blower motor that needs replacing. A well-maintained motor can last up to 30 years, so the more likely culprit is a blown capacitor, which is thankfully an easy and inexpensive fix.

A furnace capacitor works like an A/C capacitor, so you can diagnose them in similar ways. The capacitor stores energy to jumpstart the blower when the unit turns on. If it goes bad, the motor may be trying to supply energy to the blower wheel, but there's not enough boost to get it to turn.

If you hear a hum but the blower isn't turning, turn off your furnace. Using a stick, try to rotate the wheel to see if it spins freely. If so, it's likely a capacitor issue, but you'll want to run an additional test to be sure.

You can turn the unit back on and try to do a tap test to see if you can get the wheel running. It's not the safest method to test the capacitor, but it will do in a pinch if you lack the proper testing equipment.

When the motor is humming, use a stick to carefully tap the blower wheel, effectively jump-starting it. If the wheel gets up to speed and starts spinning without a problem, your capacitor is bad. If not, your motor is burnt out.

Checking and Fixing a Furnace Capacitor

Most modern furnaces have one run capacitor, a small, battery-like cylinder located near the blower motor that is held in place by a bracket.

Turn off the furnace and discharge the capacitor before removing it. Take a picture of the setup and disconnect the wiring to the capacitor, but leave the capacitor in place.

Discharge it by laying the metal of a screwdriver across the two nodes at the top of the capacitor. Make sure the screwdriver is insulated to protect you from an electrical shock.

Unscrew the mounting bracket and pull out the capacitor. You'll notice a voltage and microfarad (mfd) rating. These are important for accurately testing and replacing your capacitor.

Turn the multimeter to the capacitance setting and press the leads against the capacitor, with one lead for each terminal. After about a minute, the meter should register a number within the microfarad range on the capacitor. If the number is low or there is no reading at all, you'll need to buy a new capacitor.

The new capacitor needs to match the microfarad and voltage rating of the old one. If you're unsure about getting the right part, reach out to your parts supplier for clarity.

To install the new capacitor, insert it back into the bracket and secure the bracket screws. Use your picture to reference where the wiring is supposed to attach to the capacitor. Once installed, turn your heating on and adjust the thermostat above the room temperature to ensure the blower wheel starts turning on its own.

Whistling Sounds From Furnace

Whistling is a common and easy-to-fix furnace noise that usually stems from your ducts or your filters.

A dirty filter will whistle from air rushing through tiny holes or past small bits of debris. Replacing your filters every 30-90 days is an easy furnace troubleshooting step that can quiet it down and improve your system's efficiency.

Holes in the ductwork, especially near the blower, are a common source of whistling sounds from the furnace. Small gaps are easy to cover over with foil tape, which can handle the heat.

Ducts that are too small for the furnace can also cause whistling. Your furnace may be trying to suck in more air than the ducts can handle. If your furnace is the right size for your house, you'll need to replace the ducts with larger pieces.

Metallic Banging Sound, Scraping and Screeching

If you hear a loud scraping or metallic banging sound from furnace components instead of ducts, it could be a loose blower wheel. You can correct this by tightening the blower wheel with a wrench. If there is any damage, however, you may need to replace the blower wheel entirely.

Keeping your blower assembly well-oiled is an essential aspect of a quality furnace maintenance routine. A blower motor with dry, worn bearings may bounce during operation, creating noise and adding wear to the unit. You might even hear truck or train sounds from the furnace if the bearings get severely worn.

Along with your motor bearings, your system belts need to be well-lubricated as well, or they'll end up screeching eventually. If your blower doesn't have a direct drive motor, you need to check for any frays, breaks, or misalignments in the pulley system connecting the motor to the blower wheel.

Make sure your motor is mounted correctly, tightening any loose bolts with a wrench. If your pulleys are out of alignment, you can remove and replace them. When you've detached the pulley, you can also replace any frayed or worn belts.

When everything is secure, locate the lubrication points on the system and apply a few drops of lightweight oil. Reference your manufacturer's manual to find the ports and ensure you don't over-lubricate the motor.

Rattling Sounds

A rattling noise is usually from a loose panel or screw on your furnace. While the unit is running, try to locate the source of the rattling. Once you find it, turn off your furnace and tighten the loose part with a screwdriver.

Rattling sounds can also come from loose ductwork, which you may be able to identify just by walking around your house. If you hear rattling or vibrating ducts, you can secure them by adding more screws or duct tape to connections.

Cracked Heat Exchanger

If the rattling sound doesn't start until a few minutes after your furnace kicks on, it's probably a loose screw or connection somewhere. However, if you hear rattling just a few seconds after your furnace starts, you could have a more serious issue — a cracked heat exchanger.

The metal casing of a heat exchanger expands as it warms up, so any small cracks will widen as well, which causes them to make a rattling sound. When you hear rattling when the unit kicks on, turn your furnace off and inspect for any cracks. If you find any, you'll need to install a new heat exchanger before you turn it back on.

Don't Wait to Fix Your Furnace

When you have to ask, "Why is my furnace so loud?" it's crucial to get an answer as soon as possible. It may be something as simple as a dirty filter, or it could be as serious as bad wiring and a potential fire hazard.

No matter what, you can be sure that your system isn't running efficiently when you start to hear a noisy furnace. The faster you fix it, the more you'll save in time, money, and comfort.

The experts at Technical Hot and Cold in Michigan are dedicated to getting you the heating system parts you need at a great price. We supply a comprehensive catalog of furnace replacement parts and professional advice to ensure you have everything you need. If you have questions about how to fix a noisy furnace, contact our team today.

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