AC Unit Humming Noise: Causes, Solutions, and Prevention Tips

Is your a/c making a humming noise? This is something you’ll want to address before it turns into a big problem. But, why is the a/c unit making that humming sound in the first place?

The most common reason an a/c unit makes a humming noise is a bad motor that may have been running non-stop. This has caused it to freeze. You will need to allow it to thaw before running it again to avoid further problems. However, the noise could also indicate a problem with one of the parts, such as a faulty fan motor or capacitor.

A humming sound is usually an indicator that something is wrong. If your a/c unit is making this sound, you’ll want to find out why so you can fix it. Solving the problem now will prevent the a/c from breaking down in the future. Read on to learn what you need to do.

Troubleshooting An A/C Unit That’s Making A Humming Noise

There are several reasons why an a/c unit could start to make a humming noise. Once you figure out why it’s making that sound, you can apply the right fix. Below are some of the top causes of a humming air conditioner. 

Bad Fan Motor

The most common reason for a humming a/c unit is a bad fan motor. This can happen for a number of reasons, including debris buildup and loose parts. Don’t worry. This doesn’t mean your a/c is toast. Homeowners can replace the faulty motor to keep the a/c running efficiently.

However, it’s strongly recommended to contact an experienced HVAC professional to replace an a/c motor. You should only attempt to replace the bad fan motor if you have the correct safety equipment and are familiar with the tools needed for the job.

  • What you need: wrench, replacement motor
  1. Turn off the power to the a/c unit. 
  2. Take note of where the wires are connected.
  3. Use a wrench to unfasten the bolts that are connecting the fan and motor to the a/c.
  4. Remove the fan blade. You may have to twist and tug to successfully remove it.
  5. To install the new fan motor, reverse the steps mentioned above. Always ensure that the new fan blade is centered in the fan motor’s opening. 
  6. Turn the power back on once everything is fastened in place.

Bad Capacitor

A bad capacitor can cause a humming noise, among other problems with the a/c unit. If your a/c has a bad capacitor, you will notice that the fan won’t be running. 

Before we cover how to fix this problem, you might be wondering what the capacitor is. This is the cylinder-shaped object that’s on the outside section of the a/c. It stores energy to keep everything running efficiently. Now that you know what it is, let’s take a look at how you can repair it.

  • What you need: screwdriver or stick (something long and thin)
  1. Make sure the a/c unit is turned on.
  2. Use the stick to push the fan blades to get them to move again. This is a manual way to get the fan capacitor to work again.
  3. If this doesn’t work your capacitor may have died. In this situation, you will need to have it replaced professionally.  

Faulty Relay Switch

A faulty or damaged relay switch can cause a humming sound in the a/c. This is a switch that’s controlled by the thermostat. It’s important because it communicates to the a/c motor when the house needs to be cooled.

When this happens, you’ll likely notice warm air blowing out of the a/c along with the humming or buzzing sound. If this occurs, the first thing you should do is inspect the relay switch before assuming you need to replace it.

  • What you need: compressed air, a small soft bristle brush, replacement switch (only if the switch is damaged)
  1. Turn off the power to the a/c. 
  2. Inspect the relay switch. If the relay switch is dirty, follow the next step. If the relay switch looks burnt or damaged, jump ahead to step 7.
  3. Use the compressed air to get rid of any dust, dirt, or debris that had built up on the switch.
  4. Inspect for loose wiring.
  5. Use the brush to clean off the electrical components of the switch. 
  6. Turn the power back on and see if the a/c is functioning normally. If not turn the power back off, and move to the next step.
  7. Take a picture of the old relay switch before removing it. This way you will know exactly where the wires connect.
  8. Drain the capacitor. 
  9. Disconnect the faulty switch carefully so you don’t damage the wires.
  10. Get the new switch ready to install. Follow your picture to ensure you connect the wires to the correct terminal.
  11. Power on the a/c once everything is connected to see if the noise is gone.

A/C Unit Is Frozen

An a/c unit that is frozen will make a loud humming sound. There are a number of reasons why this can happen. However, the top one is when the a/c is running 24/7. That’s why it’s most likely to happen during a heat wave.

There’s a very easy solution to fixing a frozen a/c unit. All you need to do is turn it off. An a/c unit that runs all the time will have ice build up inside. Turning it off gives the unit time to thaw. After a few hours, when you turn it back on, the sound should be gone.

However, before we go onto the next troubleshooting suggestion, there’s one more thing that could cause an a/c to freeze. If the air conditioner is out of refrigerant, it can cause the coils to freeze. Check the refrigerant levels and refill it if it’s running low.

Loose Internal Parts

Several of the internal parts of an a/c unit move, which causes them to become loose. This is a common culprit of the humming sounds and even possible light flickering due to your AC drawing power. For homeowners, this can be tricky to troubleshoot because there are so many different parts.

It’s essential to have regular HVAC inspections. During these inspections, a professional will be able to spot any loose parts and fix them immediately. 

Always turn the power to the a/c off before attempting to tighten any parts yourself. Carefully inspect the a/c to find any loose screws. Tighten them as needed. However, when it comes to other parts in the a/c, you should contact a professional to be addressed because it’s a safety hazard if you’re not familiar with the parts.

How Expensive Is It To Fix An A/C?

It costs anywhere from $75 to over $6,000 to fix an a/c that has been making a humming noise. The costs associated with this reflect the parts you need to buy and whether or not you need to hire a professional, among several other factors. Some of the main things that will affect the cost are:

  • Type of a/c unit
  • What replacement parts are needed
  • What type of repair work need (DIY vs professional repair)

Another thing you should consider about the overall cost is how severe the damage is. If the noise is new, there’s a good chance you could get away with fixing the a/c for less than $100. However, if there is a lot of damage done, you can expect a much higher repair bill.

To give you a better idea of how much it is to fix an a/c, let’s look at the average costs of the replacement parts.

PartAverage Price
Fan motor$400 – $450
Capacitor$220 – $500
Thermostat$120 – $550
Compressor$1,00 – $2,500
Relays$80 – $300
Condenser coil$550 – $1,100
Evaporator coil$200 – $6,000
Contactor$175 – $400

Keep in mind, these are just the costs of certain replacement parts. If you contact a professional, you will also have to include the cost of labor. Most HVAC professionals charge customers $75 to $150 per hour. However, most reputable HVAC professionals will provide a quote or estimate so you know what to expect for the budget before hiring them. 

When Is It Time To Call A Professional?

Are you wondering if the fix is DIY or in need of a professional? The a/c unit has a lot of parts. Depending on the fix, you could be in over your head. This is when you should trust a professional to take over. Working on an a/c can be very dangerous if you aren’t familiar with them.

A professional is familiar with the entire a/c system. They know what the parts are and how to detect if there’s a problem. On top of that, they have critical thinking skills used on the job to ensure they fix it properly. Your a/c issue will be diagnosed and fixed correctly without any second guessing.

The one downfall of a professional is the price. You can expect it to cost a lot more than a DIY fix. While that may be a big factor to consider, there are a few things you should think about when it comes to a professional fix.

  • Quick fix. When you fix the problem yourself, there is a lot of guesswork involved. You could troubleshoot the problem incorrectly. This means that it will take longer to fix and you could spend money on parts you don’t actually need.
  • Warranty. Most reputable HVAC professionals place a warranty on their work. If that humming noise comes back, so will the HVAC professional to assess it. You won’t pay for the second trip. 


If your a/c unit is making a humming noise, you should troubleshoot the problem right away. This prevents the problem from getting worse and leading to a breakdown. One of the first things to consider is how long the a/c has been running. It may be frozen and needs to thaw.

However, if this isn’t the case, the noise indicates an issue with one of the parts. There could be loose internal parts or damage to the fan motor or capacitor. While many of the problems can be solved DIY, you should always contact a professional for extensive repairs.