Why is My AC Making Loud Noises when Starting

Air conditioners get damaged, and after some time, you may notice some loud, strange noises when your unit starts up or shuts down. What could be the reason?

If you are wondering why your air conditioner makes loud noises when starting or shutting off, then you’ve come to the right place. In this article, you will discover;

  • The potential causes
  • Repair tips
  • Common AC noises and what they may signify. 

Causes for Air Conditioner Making Loud Noises When Starting or  Stopping

If the air conditioner makes loud noises when it starts up, it could be a faulty compressor motor or a broken fan motor. A creaking noise can also be heard in the AC or HVAC system when the sheet metal ducts contract and expand.

However, the factors also depend on whether you have an outdoor unit or an indoor device.

So, let’s dig deeper into some of the causes for both types of air conditioning devices.

1. Indoor Air Conditioner Makes a Slamming Noise

Here are some causes and tips on how to repair the AC.

  • The Air Filter Is Hitting the Return Grille

The spot where warm air from your house enters the AC for cooling is known as the return grille. First, the air has to pass through a filter that removes contaminants like dust before it arrives in the actual AC system.

A loud slamming noise can sometimes occur when you turn on a ceiling AC. When the unit starts working, the air filter may get sucked up to the metal enclosure’s top. The moment the AC is turned off, it stops pulling in any air, and it ends with the filter dropping onto the return grille. This is when you hear the loud slamming noise. 

Solution: You need to contact an HVAC technician to check the return grille. The air conditioning expert will see if the air filter is well-fitted and whether it has the right size for your system. If the air filter is not responsible for the issue, then the return ducts might cause the problem.

  • The Blower Fan Is Loose

Cool air is sent back into your house after the blower fan pulls warm air in for cooling.

The fan mounts can become loose, making the blower wheel bang against the metal casing when the AC slows down and stops.

Solution: A HVAC system technician can help inspect the issue and tighten loose connections.

  • Issues with Dampers in Your Home

Some ACs have dampers that help to control the cooling airflow going into different home sections. 

Therefore, slamming noises may be caused when you shut down the unit and the dampers are closing.

Solution: Calling an expert for repairs is the right approach to resolve the problem.

  • The Air Filter Is Too Thick

Another reason why your indoor unit makes a loud noise is the air filter being too thick. 

Some people go for high-MERV filters, but while they are great for air quality, they decrease airflow.

Solution: Going with a lower MERV rating air filter is much better.

  • The Fan Cage Is Damaged

A damaged fan cage might also grind against the fan blades and cause a loud noise. This can result in more damage to the unit.

Solution: In this case, you should turn off the device and call a professional for AC repair.

2. Outdoor Air Conditioner Makes a Slamming Noise

Outdoor ACs also have their own issues that cause noise, especially since they go through temperature changes.

  • The Compressor Pump Support Springs Are Worn Out or Broken

There are compressors built with springs to help absorb the pump vibrations. 

When a spring breaks, there will be wobbling in the housing and loud noise when the HVAC system is turned on or off. 

Solution: If you don’t want this to go on until the compressor fails, you should replace it or buy a whole condensing unit.

  • The Outdoor Fan or Motor Is Broken, or One of Them Needs Adjustment

When the outdoor fan works at full speed, it spins quietly. Because of this, you may not be aware of this issue. However, the fan is slower when you turn on or shut down the AC. In these moments, when the fan is wobbling, it produces a banging noise.

It’s easy to notice when the fan is wobbling, so check it out and listen.

Solution: It may be hard to fix the issue alone if you’re not an expert, so call an HVAC expert to inspect it and repair it.

  • Ducts Expanding and Contracting

If your ducts are made of sheet metal, their expansion and contraction can cause unbearable noise. 

This happens due to temperature and pressure changes. 

Types of AC Unit Noises and What They Might Mean

A technician inspecting an outdoor air conditioner

  • “Wooshing” Sound

The “wooshing” sound is most commonly heard during winter due to the heat pumps. 

These heat pumps use reversing valves to switch the refrigerant’s flow between the cooling and heating modes. 

When the heat pump goes into defrost mode, the valve shifts, causing the “wooshing” sound.

  • “Tinny” Sound

The “tinny” sound usually occurs after the “wooshing” one, when the compressor has a louder sound than usual. 

This happens as the refrigerant pressures equalize after shutdown, and noise can be heard.

  • “Back-Peddling” Sound

You can hear the “Back-peddling” sound when shutting off or turning on the AC.

Newer “scroll” type compressors, in particular, make this sound. 

You will hear the noise at shutdown, whereas at start-up you’ll hear an “out-of-balance washing machine” noise.

  • “Buzzing” Sound

Buzzing sounds are usually heard from outdoor units. Basically, the reversing valve solenoid coil is the culprit here, and the sound can be heard even then the air conditioner is not running. 

A low voltage device may do this occasionally, and some are much louder than others.

  • “Metal-Hitting-Metal” Type of Sound

A “metal-hitting-metal” type of sound is caused by the fan blades hitting the tubing, a wire, or ice. 

In that situation, please turn off the air conditioning to avoid further damage and call a professional to inspect it.

  • “Vibration” Type of Noise

One of the most common air conditioner sounds is vibration. They are also the hardest to solve. 

In some cases, it happens due to the refrigerant piping that is slightly strapped to the studs or joints in the house. 

At times the unit needs some rubber insulation pads under it. 

The unit itself may also cause vibrations sometimes.


AC Maintenance is key. Still, they can malfunction sometimes. 

Knowing why your air conditioner makes a loud noise when starting or shutting off, plus their causes can help you develop the right solutions.

Keep in mind the differences between indoor and outdoor devices and always call on professional services to solve the problems as soon as possible. 

If you have any other questions, leave us a reply in the comment section.


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Ronald Watford, the owner of Quality Home Air Care, is a qualified HVACR technician and manages the team of expert writers on this site. He believes that educating homeowners about HVAC systems is one of the most impactful aspects of his job.