Why is My Furnace Making Loud Noises When My AC is On?


Have you noticed strange noises coming from either your furnace or AC unit when the other is running?

Furnace and air conditioning systems often work in unison to regulate indoor temperatures. If you begin to hear unknown noises coming from these units — such as banging, squealing, or a variety of other sounds — it is best to investigate the source of the noise as soon as possible. 

There can be many potential underlying causes at the root of your furnace or AC noise problem, including everything from loose parts to malfunctioning equipment. Getting to the bottom of the noises you are hearing is the key to ensuring your furnace and AC are functioning properly and efficiently.

Moreover, identifying strange AC or furnace noises is also crucial for ensuring your home’s safety. 

In this article, we discuss seven of the most common causes of noise in furnaces and AC units, as well as how to identify and resolve the underlying issues in your equipment.

Keep reading to find out when to turn to AC and furnace professionals for help! 

5 Causes & Solutions for Dealing with Furnace & AC Noises

When you begin to notice a strange noise coming from your AC or furnace, the first essential step is to determine what type of sound the noise is producing.

In general, we can identify seven main types of sounds:

  • Banging
  • Clunking or Rattling
  • Clicking
  • Buzzing or Humming
  • Squealing

Each of these sounds comes with a variety of potential causes, varying in severity.

Let’s examine what can cause these noises in your furnace or AC — and what you can do about it: 

1. Banging Noise in Your AC or Furnace

If your AC or furnace is making a banging noise, you should first determine when that banging noise is occurring. In cases where the banging noise can be heard when the unit first starts and stops blowing air, this can typically be explained as normal expansion and contraction.

However, if the banging noise occurs while the furnace or AC is running, this can indicate a problem.

Here are the potential causes behind a banging noise in your AC or furnace:

  • Dirty Burners: Over time, your furnace burners can build up a layer of filth and debris brought into the system by air. This build-up can cause functional problems when the burners ignite, resulting in a banging noise.  Resolving this issue requires your burners to be professionally cleaned. To prevent this in the future, schedule annual furnace maintenance appointments. 
  • Fan Issues: Like with furnace burners, fans in a furnace or AC system can build up debris over time. If this debris is allowed to build up too much, this can result in blockages that create banging sounds as your fans struggle to function properly. Once again, the best solution for this problem is to have your fans cleaned and unclogged, with annual maintenance performed. 
  • Broken Components: If a component of your furnace or AC unit becomes broken or loose, it can create a banging noise as it moves around in the system. This is typically a louder banging sound that is coupled with other sounds, such as rattling or vibrating. If you confirm the sound is not due to a clog or build-up, get in touch with a repairman as soon as you can. 

2. Clunking or Rattling Noise in Your AC or Furnace

A clunking or rattling sound in an AC or furnace is less sharp than a banging sound and can indicate a variety of functional problems. 

Clunking or rattling sounds are often heard in AC units, with the following common causes: 

  • Contracting & Expanding Air Ducts: A fairly common cause of clunking noises in AC is the contraction and expansion processes in the air ducts. Higher temperatures cause the metal components of your air ducts to heat up and make noise as they knock against other components, like the vents. Fortunately, this process is normal and does not generally require repairs
  • Broken Fan Belt: In more serious instances of a clunking noise inside an AC unit or furnace, you could be dealing with a broken or cracked fan belt. A broken fan belt produces a recurring, distinct clunking noise when the unit or furnace is running. Repairing or replacing a fan belt requires you to remove the unit panel. Unless you are familiar with AC or furnace repairs, the best practice is to call in an HVAC specialist for your fan belt repair needs.
  • Dirty Blower Wheel: As with the burners and fans in a furnace or AC unit, a blower wheel can gather dirt and debris over time which leads to a build-up. This can cause your blower wheel to become unbalanced and malfunction — and, in worst-case scenarios, even explode — leading to costly repairs. Avoiding this problem requires changing your filter regularly, with recommended filter changes at least every three months. 

3. Clicking Noise in Your AC or Furnace

Unlike a banging or clunking noise, a clicking noise is a subtler sound that can be incredibly irritating once you notice it. Before you can identify the underlying cause, it is important to try to pinpoint the source of the clicking as closely as possible.

Once you have figured out where the clicking is coming from, you can narrow down the cause of the issue based on the following potential problems:

  • Faulty Ignition: If the clicking is coming from your furnace’s ignition, this can be a major sign that your ignition is malfunctioning. This malfunction can often be attributed to a clogged sensor or an unopened gas valve. The clicking noise comes from your unit’s recurring attempts to ignite. 
  • Powering On & Off: After evaluating your system, if you notice the clicking only occurs as the unit powers on or off, this can be a normal sound that indicates your system is communicating with its thermometers and other sensors. In general, this type of clicking is no cause for concern.  However, in some cases, this could indicate a malfunctioning thermostat — if this is the case, the clicking will likely be persistent and the unit may not heat or cool properly. 
  • Fan Obstruction: A recurring clicking noise can be produced when a fan blade is obstructed in some way within your unit. If left unaddressed, this can cause more serious damage to your blower fan or fan belt. Your best bet is to contact an HVAC professional for assessing and repairing issues related to the fan system. 

4. Buzzing Noise in Your AC or Furnace

A buzzing noise is similar to a whirring noise and can often be indicative of a mechanical or electrical issue in HVAC equipment. In almost all instances of a buzzing noise in your furnace or AC, it is best to get in touch with an HVAC professional as soon as possible.

Here are some possible causes of a buzzing noise in your unit: 

  • Loose Components: Over time — or in instances of improper installation or repairs — components of your HVAC system can become loose. The two most common components that can produce a buzzing sound include a loose transformer box and loose ductwork. In the case of the former, this can often mean the transformer box needs to be either re-tightened or replaced. As for the latter, the ductwork connections should be inspected by a professional. 
  • Faulty Capacitor: A capacitor is the component of your system that is responsible for storing electricity and starting up the motor. A buzzing noise in your HVAC system can indicate a problem with the capacitor that may require repairs or replacements. 
  • Blower Motor Malfunction: Blower motors are used to circulate air throughout the ducts in your home. If this motor is damaged or malfunctioning in any way, this can produce the buzzing sound you hear while your system is running. Blower motors are a vital component in any HVAC system and should be assessed by professionals if a buzzing sound occurs. A humming noise can also be attributed to an unbalanced blower motor that is beginning to fail. 

5. Squealing Noise in Your AC or Furnace

Squealing noises coming from your AC or furnace can be particularly loud and obnoxious.

In almost all cases of a squealing noise, the underlying cause is a loose fan belt. Fan belts can become worn out over time, causing them to loosen. This can also produce a smell of burning rubber — a scent that should always be taken seriously when it comes to the integrity of your fan belt. 

If a fan belt has simply come loose, you can generally fix this on your own by tightening the screws attaching the belt to the system. Otherwise, you should contact an HVAC professional for help replacing a worn-out or damaged fan belt. 

Final Thoughts: When to Call Your Local HVAC Pro

If you want to make sure your HVAC system is in great working condition, you should always give your local HVAC experts a call whenever you hear a suspicious noise.

As you get more comfortable with at-home maintenance, you may be able to identify HVAC sounds with greater precision. In turn, you can have greater confidence when explaining a noise to an HVAC team and what potential causes you believe may be behind the noise.

To keep your HVAC system in the best condition possible, make sure to schedule routine maintenance for your system, including cleanings and repairs.

Looking for an HVAc Professional in Brentwood, TN?

We are the right choice for all your HVAC needs! Here at Rapid Response, we understand that no two customers are the same and that is why our team of trained professionals will go above and beyond to meet the customized cooling needs of every customer. By choosing us you will receive fair rates, quality work and outstanding service.