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Why Does My AC Make a Slamming Noise When Shutting Off

Mar. 24, 2020


A loud, slamming noise is discomforting—especially when you think it’s coming from your AC.

While it’s hard to know exactly what could be causing the loud sound without hearing it ourselves, we can give you some general information about what’s causing the problem.

The first question you’ll need to ask yourself is: Where do you hear the noise coming from?

If you hear the slamming sound coming from inside your home, your AC may have one of the following issues:
  • The air filter is hitting the return grille
  • The blower fan is loose
Hear the slamming sound coming from outside your home? If so, your AC may have one of the following issues:
  • The outdoor fan is hitting metal
  • The compressor springs are broken
We’ll go into more detail about each of these problems below. We’ll also tell you right off the bat, to fix most of these problems you’ll need to hire an HVAC professional.

Need an AC repair? Let a trusted New Jersey technician help.

Schedule appointment today
 

Indoor slamming sound problems

Hear the slamming sound inside your home? You may have one of the following problems:
  • The air filter is hitting the return grille
  • The blower fan is loose

Problem #1: The air filter is hitting the return grille

The return grille is the opening where warm air from your home enters the AC to be cooled. Before the air reaches the actual AC system, it passes through a filter to remove dust and other contaminants.

Return grille located on ceiling
Return grille located on the ceiling

In some New Jersey homes, the return grille (vent) is located on the ceiling. When the AC first turns on, sometimes the filter gets sucked up to the top of its metal enclosure. After the AC stops pulling in air and shuts off, the filter drops onto the return grille, creating the loud slamming noise.

Solution: An HVAC technician will need to inspect your return grille and see if the air filter is fitted properly and that it is the correct size for your system. If the issue is not with the air filter itself, it could mean the return ducts are undersized. 

With undersized ducts, air can travel faster than normal, which causes the filter to get sucked up then drop when air stops flowing. A professional can determine if this is the problem and modify your ductwork to stop the air filter from getting sucked up.
 

Problem #2: The blower fan is loose

The blower fan pulls in warm air to be cooled, then pushes cool air back into your home.

Over time, the fan mounts can become loose which causes the blower wheel to bang against its metal casing when the fan slows down (i.e. when your AC stops). This banging sound can sometimes be described as a “slamming” sound. 

Solution: An HVAC pro will need to inspect the blower wheel assembly and tighten any loose connections.
 

Other causes of a slamming noise indoors

The following scenarios could cause a slamming noise when your AC shuts off, but they don’t necessarily mean something is wrong with your air conditioner. We’ll explain.
 

Cause #1: A damper is closing

Some homes have dampers installed in the ductwork to direct air to certain rooms in your home. The slamming noise you hear may simply be a damper closing.

If the slamming noise is too distracting, you can have a professional inspect the damper to make sure it’s installed correctly.
 

Cause #2: Ductwork is expanding and contracting

If your home has sheet metal ducts, after the AC shuts off you may hear your ducts expand and contract from pressure/temperature changes. 

This sound can often be described as slamming, but some homeowners refer to it more as a “popping” sound. 

A certain amount of this popping sound is normal, but if the sound is loud and more frequent than normal, you may want a professional to inspect your ductwork. They can check and make sure the ducts are reinforced and sized properly.
 

Outdoor slamming sound problems

Hear the slamming sound coming from the outdoor part of your AC? If so, you may have one of the following problems:
  • The outdoor fan is hitting metal
  • The compressor springs are broken

Problem #1: The outdoor fan is hitting metal

The fan inside your condenser
The fan inside your condenser

The outdoor unit has a fan to help disperse heat absorbed by your AC system. Due to general wear and tear, sometimes the fan can become loose and hit metal parts in the outdoor unit when the AC shuts off, creating a loud slamming sound.

Solution: An HVAC technician can inspect the outdoor fan assembly and make sure it’s tightly secured and operating properly.
 

Problem #2: The compressor springs are broken

The compressor is a hermetically-sealed cylinder, located in the outdoor unit. It is the part that circulates refrigerant in your AC system. (Refrigerant is a liquid/gas that cools your home’s air by removing heat.)

The compressor of a central air conditioner
The compressor of a central air conditioner

Inside the compressor casing is a complex series of mechanical components, including refrigerant pumps. These pumps are usually supported by springs, which can break over time. When a support spring breaks, the loose pump can bang against the compressor case, creating a loud slamming noise.

Solution: You’ll need to contact an HVAC professional for assistance. Unfortunately, fixing a compressor spring in the field is nearly impossible, which means you’ll need to have your compressor replaced (a very expensive repair). 

The HVAC technician you hire can help you determine if your compressor is still covered under warranty and what repair options you have. 
 

Need an AC repair from a New Jersey tech?

If you’re tired of the slamming noise and want a professional to fix it ASAP, give Air Experts a call. We’ve served New Jersey residents since 1994, providing 5-star AC repairs and exceptional customer service for every job.

Schedule appointment today
 

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