Furnace Making Humming Noise? 5 Common Causes (+ Fixes)

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A furnace is an important house fitting that is necessary for many living spaces, especially those in cold environments, as it makes these spaces warm and comfortable. However, when your furnace makes a humming noise every time you run it, you are more likely to be troubled than comfortable.

The most common source of humming noises in a furnace is a defective blower motor. The blower motor circulates heated air from the furnace to the vents that distribute the air to the rest of the house, and thus it is the heart of the furnace.  

The rest of this article provides a detailed examination of issues related to the topic at hand, including how a furnace works, common causes of humming noises in a furnace, and how to fix humming-related problems in a furnace.

How a Furnace Works

Most furnaces comprise five main components: burner, heat exchanger, blower motor, thermostat, and ventilation pipes.

Functions of the Main Components of a Furnace

  •  Burner: this is where the mixture of air and fuel is burned to produce heat.
  • Heat exchanger is a thin sheet of metal that lies between the blower and the combustion chamber. The combustion chamber heats the exchanger, and it, in turn, heats the breathable air that passes through the furnace before this air goes up the ducts.
  • Blower motor: pushes the heated air past the exchanger from the furnace to the ducts.
  • Thermostat: determines the amount of heat provided by the furnace.
  •  Ventilation pipes: remove toxic byproducts of the burned fuel from the house.

The furnace is activated when the thermostat determines that the air temperature in the house is less than the preset temperature. The thermostat signals the ignitor to turn on, upon which the ignitor receives an electric current that lights the burner under the combustion chamber. The burner then heats the metal heat exchanger that warms the cold air in the house.

Meanwhile, the toxic combustion gases produced in the combustion chamber are removed from the house through the ventilation pipes. Here is a video that explains how the components of a furnace work together to keep your house warm:

Common Causes and Fixes of Humming Sounds in a Furnace

Cause #1: A Defective Blower Motor

As I had mentioned before, a faulty blower often is the most common cause of humming noises in a furnace. Such a blower can exhibit one or more of the following features:

  • Humming noises
  • Increased energy consumption
  • Overheating
  • Airflow problems

If the blower motor makes humming noises after it has slowed down, then the culprit could be a faulty capacitor. Suppose you suspect that the problem is a faulty capacitor, and you are comfortable with operating the components of the furnace without the help of an expert technician. 

In that case, you can use a multimeter to test the capacitor to ensure that it has the correct capacitance rating. If the capacitor fails the rating test, you should replace it.

You can also tell that a capacitor is bad if its casing is damaged in any way.

Alternatively, the blower motor can hum when the blower is on or off. In such cases, the problem component is the blower pilot light or the gas burner. To fix this problem, you can try to adjust the pilot light to an appropriate level or clean the burner.

Finally, the hum in the motor could simply be the motor telling you that it needs lubrication. To oil, the blower motor, refer to the user’s manual for information on the specific oil to be used and the location of the oil ports. Then, squeeze a few drops of oil in each port.

Cause #2: Loose Components

The inside of a furnace comprises many components that work together to ensure that the equipment performs its functions effectively and efficiently. With time, the vibration of the furnace can cause some of these components, such as screws and bolts, to loosen and cause humming noises in the furnace. To fix this problem, ensure that all loose components are secured.

Cause #3: Transformer

The furnace transformer supplies power to the system. It naturally emits a low hum during its operations; however, if the humming becomes too loud, you can troubleshoot the problem by checking whether the transformer box is properly mounted. If it is loose, fasten the loose screws. Sometimes, the hum in the transformer can signify a fault within it. In such a case, it is best to consult an expert technician to remedy the problem. 

Cause #4: Ducts

A partial or total blockage in the return ducts can cause poor airflow that results in humming noises. The return ducts suck in air from the house and send it to the furnace, where it is filtered before heating. The air taken in by the ducts is naturally dusty; the dust collects and accumulates in the ducts over time, finally clogging them.

Clogged ducts reduce the flow of air in the furnace, with the effect that the furnace becomes overheated, thus leading to multiple overarching problems. For example, the exchanger can overheat and crack, thereby causing spillage of toxic gases into the house, or huge energy bills can be accumulated because the furnace has to work harder and use more energy to pull the air back into the furnace.

A damaged heat exchanger will often have cracks on it, emit a strange smell similar to that of paint varnish, or have soot stains on its sides. Because heat exchangers are expensive components, it is often better to replace the entire furnace instead of only replacing the heat exchanger.

To solve the problem of reduced airflow in the return ducts of the furnace, you can do the following:

  • Open all the vents to improve the flow of air within the ducts.
  • Regularly clean your vents and ducts.
  • If the above-mentioned steps do not help in reducing the humming noises, consult an expert technician.

Sometimes, the hum in the ductwork can be the result of vibrations caused by the air as it moves through the ducts. This problem can be solved by securing the ductwork more firmly to its support system.

Cause #5: Dirty Burner

An accumulation of dirt on the burner can interfere with the airflow to the burner, thereby creating humming noises. Also, blocked burners affect the heat performance of the furnace and may even prevent a furnace from starting when turned on. To solve this problem, the burners should be cleaned or replaced, whichever is appropriate.


A humming noise from a furnace may not necessarily signify a problem; some furnace components characteristically hum while in operation, for example, the furnace transformer. Therefore, it’s important to troubleshoot humming noises in the furnace to determine the culprit components.

The most common cause of humming noises in a furnace is a faulty blower motor. If you are well versed with the inner workings of your furnace, you can attempt to remedy the problem in the motor by oiling its ports or replacing its capacitors. Otherwise, you can enlist the services of an expert technician to help you solve the problem. However, most of these problems can be avoided by following a regular furnace maintenance schedule. 

Other problems that can cause a furnace to produce humming noises include loose furnace components, such as screws and bolts; a defective transformer; blocked ducts; loosely secured ductwork; and dirty burners.

Nicole Sutton

Nicole is a writer from Missouri who enjoys horseback riding, writing in her journal, and watching reruns of Friends.

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