December 10, 2022

Furnace Repair in St. Louis, MO

Furnace Making Loud Noises? Here’s Why and What to Do

Furnaces always make some noise whenever they are running. However, any loud or unusual noises are usually a sign that there is some issue with either your furnace or the blower fan. If you only hear the strange sound occasionally and only ever for a short time, it may be nothing to worry about. However, if the noise persists, it is definitely something you will want to get checked out as soon as possible. Let’s take a closer look at some of the most common furnace noises, what they mean and how they can be fixed.

Booming Noise When Furnace Starts

There are some cases where your furnace will make a loud booming noise when it starts that almost sounds like a small explosion. If you ever hear this noise, you should turn your heating off until you can have an HVAC technician inspect and clean the furnace. This booming sound means that your furnace burners are overly dirty and need to be cleaned.

Dirty burners often cause a short delay before the furnace ignites. This leads to some gas flowing out into the combustion chamber and basically results in a small fireball being created once the burners finally light. The issue usually isn’t severe enough for the furnace to ever have a chance of exploding, but it can still cause major damage to the unit, so it is something you should get taken care of immediately. You can also easily avoid this problem — and most other unusual noises and furnace issues — by making sure to have your heating system professionally maintained every fall.

Clicking Noises

Clicking sounds coming from the furnace are quite common and can be caused by a variety of different issues. One potential cause is a malfunctioning electric igniter. Most furnaces no longer use a standing pilot light and instead use either direct-spark or hot-surface ignition. A direct-spark igniter is similar to the electric igniter used on most barbecue grills, and it works by creating a spark that ignites the gas flowing through the burners. If the igniter fails, it will make a loud clicking noise every time it attempts to spark and your furnace won’t ever start.

Another possibility is that the flame sensor is dirty. If your furnace starts and makes a clicking noise for a few seconds before shutting off again, this is likely the culprit. The flame sensor is a safety feature that prevents gas from flowing into the furnace if it isn’t lit. If it doesn’t immediately detect a flame, the igniter will continue creating a spark even though the furnace is lit. If the sensor still doesn’t detect the flame within a few seconds, it will then close the gas valve, and the furnace will shut down.

You will also hear a clicking noise coming from the igniter if gas ever stops flowing to the furnace for any reason. In this situation, the igniter will continue creating a spark, but nothing will ever happen since there is no gas flowing into the furnace.

One final possibility is that the start capacitor that powers the blower fan is failing. The capacitor stores an electrical charge that it releases to help the blower motor start up. When the capacitor begins to fail, it will almost always make a loud clicking noise when the fan attempts to start. If you hear this noise and the fan doesn’t immediately start when the furnace does, it means that the capacitor is bad and will need to be replaced.

Squealing or Screeching

Any loud squealing or screeching sounds are also related to the blower fan. If your HVAC system is fairly old, the fan motor is most likely belt-driven. In this case, the noise means that the belt is worn out or misaligned.

Newer HVAC systems always use a direct-drive motor instead, and this type of motor is powered by bearings. When the bearings start to wear out, they will usually produce a loud squealing sound. This is something you should get taken care of immediately. The bearings on the motor can be replaced, but if you wait too long, it could result in the motor burning out, which will be a much more expensive repair.

Grinding or Scraping

If you hear any scraping sounds or something that sounds like metal grinding on metal, it typically means that the blower wheel has come loose from its housing. When this happens, it can cause the wheel to continually scrape against the housing. This is another problem you need to get checked out immediately or else it could ruin the blower and force you to have it replaced. As with most other sounds, annual maintenance should prevent this issue as the technician will always check that the bolts and screws are tight and the blower is properly aligned.


Whistling sounds coming from your furnace or ductwork are a sign of reduced airflow. This most commonly happens because the air filter is dirty and needs to be replaced, and this is always the first thing to check if you hear any whistling noises. These sounds could also be a sign that your return-air vent is clogged or obstructed or that your ductwork is damaged and leaking.

Insufficient airflow can be a serious problem. Not only will it prevent the furnace from producing much heat, but it can also cause all of the heat to build up inside the unit. This can lead to the furnace overheating and automatically shutting down to prevent any potential damage. The biggest issue with overheating is that it can cause the heat exchanger to crack, which will result in carbon monoxide leaking into the ductwork and flowing out of all of your vents.


Rattling noises can result from a few different issues, so you will want to do a bit of investigating to see exactly where the noise is coming from. If it sounds like the noise is coming from your ductwork, this usually indicates that one of the ducts is sagging or has come loose from the wall or ceiling. While this isn’t usually a big issue, it can definitely be annoying.

If the rattling sounds are coming from the air handler above or next to your furnace, it means that the blower has started to come loose. If left unchecked, this could result in the blower making grinding noises as we’ve already discussed.

Rattling sounds coming from the furnace itself could potentially be a sign of a more serious problem. If you’re lucky, it may just be that the cover is slightly loose. However, this issue more commonly means that the heat exchanger is cracked. Heat exchangers most commonly crack as a result of the furnace overheating, but it can also happen simply as a result of normal wear and tear due to age.

A cracked heat exchanger is the most serious of all furnace issues due to the risk of potentially fatal carbon monoxide poisoning. For this reason, you should always leave your heating system shut off until you can have it inspected by a certified furnace technician.

If your furnace is making any loud or unusual noises, Scott-Lee Heating Company is ready to help. We service and repair all makes, models, and types of furnaces, and we also work on air conditioners, mini-splits, heat pumps, geothermal HVAC systems, and indoor air quality equipment. In addition, we do custom metalwork like copper flashing, and we can help with HVAC zoning. If you need any residential or commercial HVAC service in the St. Louis area, give us a call today.