So your AC is working fine but makes a loud slamming-like noise when it shuts off. What could be causing it? There are a number of possible causes. We’ve broken down the most common by where the noise is originating from: inside or outside of your home.
Keep in mind, though, that without hearing the banging sound ourselves, it’s difficult to correctly diagnose. So for added peace of mind, we always recommend having a professional AC tech take a look (especially if this is a new noise).
If the noise is coming from inside your home…
Complaint: It sounds like there is a door inside the ducts that is slamming shut when the air conditioner turns off.
Indoor cause #1: Air filter is slamming against the grille
Most homes in the Phoenix area have return vents located in the ceiling of the home.
And inside the grilles of these vents is your air filter.
When your air conditioner turns on, these ducts suck in air from your home. Under certain conditions, the air filter may get sucked up against the top of the duct box. Then, when the AC turns off, the filter drops to the grille, making a slamming or thumping noise.
This is not a normal operating noise and is usually a sign of one of the following underlying problems:
An air filter that is too restrictive
Some homeowners install highly efficient air filters to improve the air quality of their home. Normally, this is a good idea. But if you get a filter that is too efficient (noted by a MERV rating of 10+) and only 1-inch thick, it can restrict the amount of airflow through the filter.
As the air conditioner tries to pull more air through the filter, the filter itself can get sucked up into your ductwork.
The fix: You have 2 options:
- Replace the filter with a less restrictive, but less efficient, filter—While a basic air filter won’t improve your home’s air quality, it also won’t cause the slamming problem.
- Upgrade to an air scrubber—An air scrubber is basically a thicker (up to 6-inches) air filter, which catches more airborne contaminants (dust, smoke, etc) without restricting too much airflow.
Your air filter is dirty
When your air filter gets too dirty, the dirt restricts the airflow to your air conditioner. And just like we explained above, that can cause the filter to get sucked up into the duct as the AC attempts to pull in more air.
The fix: Clean or replace the air filter.
Your return ducts are undersized
If your return ducts are too small, the speed of the air going through the returns can get powerful enough to suck an air filter up to the duct.
The fix: You’ll need an AC repair company to run new ductwork.
Indoor cause #2: A closing damper
Some air conditioning systems have dampers in them to change the amount of airflow flowing to different parts of your home. The slamming noise may be the damper closing after the AC shuts off.
The fix: You may not need to do anything, as this could be normal operation. But if the banging is extremely loud, contact an HVAC company to check it out, as the damper may need to be repaired or replaced.
Indoor cause #3: Expanding/contracting ducts
If you have sheet metal ducts, you may be hearing creaking noises from the ducts expanding and contracting from the pressure and temperature changes.
The fix: Most likely your ducts are improperly sized for the amount of air going through them, which is what’s causing them to “pop” as your air conditioner turns on and off. Or the ducts might not be reinforced properly. You’ll need to have an AC company find and fix the duct problem.
Indoor cause #4: Problem with the fan
There may be a problem with the fan in your indoor unit (or air handler) that’s causing the blower wheel to bang against another part of the unit. For example, the fan’s mounts can get loose, which can throw it off balance as it slows down, causing it to hit its enclosure.
The fix: The blower wheel assembly may need to be repaired or replaced by an AC professional.
If the noise is coming from outside your home…
Complaint: When your AC condenser (outside unit) shuts off, there’s a loud bang that sounds like something hit a wall.
Outdoor cause #1: Compressor’s internal support springs are shot
The compressor is the part of your outdoor unit that pumps refrigerant to and from your indoor unit.
And inside the sealed compressor casing are many parts, including the refrigerant pump. Most of these pumps are mounted on support springs. But when a support spring breaks, the pump isn’t held evenly any longer.
This makes it sort of like a spinning top: when it’s going quickly, its momentum keeps it upright and balanced. But as it slows down it starts wobbling.
It’s the same for a compressor with a broken spring. When the air conditioner turns off, the pump loses momentum, which can cause it to knock against the case of the compressor and create a banging or slamming noise.
The fix: To get rid of the noise, you’d have to replace the whole compressor since it is a single, sealed part. And replacing a compressor is expensive!
So, if it’s not under warranty, we recommend you just leave it. The sound isn’t necessarily an indication that the compressor will fail anytime soon.
Outdoor cause #2: Fan knocking against unit
Your outdoor unit also contains a fan. And just like the indoor fan, parts can become loose. And then the fan can hit other parts of the outdoor unit, causing a slamming sound.
The fix: Depending on the issue, you may need a whole new fan assembly, or a few bolts may need to be tightened.
Find the source of your AC’s slamming noise
To figure out which of these problems is causing the slamming noise you’re hearing, you need a trained tech to listen to it. If you’re in the Phoenix area, George Brazil can help. We’ve been serving the Valley since 1955.