Air conditioners and heat pumps are known to make loud noises when starting and shutting off. The sounds are described as the AC making slamming noises, AC making banging noises and the air conditioner making thumping noises at start or stop
Why the AC makes a loud noise at start or stop?
We’ve organized the problem this way (Navigation):
Use the Navigation above to jump to indoor or outdoor, if you’re sure you now where the noise is happening. Otherwise, join us for the full journey to finding out why AC is making loud noise when it starts or shuts off.
Start and/or Stop – This guide covers noises noticeable at these two times. If your AC is making bad sounds at other times of the cycle, our Guide - Air Conditioner Making Noise – Pulsating, Humming or Buzzing? might help you solve the issue.
There are a couple things that might be happening inside at the start of the cycle. They could happen at the end of the cycle too, but this is when you first notice it, right?
Indoor Noise with Easy Fix
As we talk to homeowners and read their stories on forums and review pages, one “AC slamming sound” stands out among the rest for its frequency – and the ease with which it is repairable.
The air filter is slamming against the frame, sheet metal or the grille
Air filters are an essential part of keeping your home’s indoor air quality good through removing dust, pollen and dander.
They also protect the indoor AC coil from excessive dust and debris that will reduce energy efficiency and make the unit run harder, potentially leading to early mechanical failure.
Why the AC slamming noise at start or shutdown happens?
If the filter is loose in its setting, it can move back and forth when the powerful blower motor turns on – sucking the filter toward it at the start and “dropping” it and the end of the cycle.
If the filter frame is metal, you’ll hear a “bang.” If the frame is cardboard, it can still “thump” pretty loudly.
How to check if the filter is banging or thumping?
This is easy. Remove the filter, and run the AC for a cycle or two. Does it thump?
If so, the filter isn’t the issue. Skip down to the issue with the blower motor and fan.
If the loud sound at the start of the AC cycle or at the end doesn’t recur, the filter is the issue.
Filter Noise: Why it happens and how to fix it.
1). The most likely option is that the filter is dirty.
Rather than allowing air through it, the entire filter moves, making the noise.
Solution: Replace or clean the air filter and return it to its place.
2). The air filter is too thick.
Many homeowners are sold on using high-MERV filters, those MERV-12 and higher, to reduce pollutants in the air.
While these do improve indoor air quality, they also reduce airflow, which can have the same impact as the filter being dirty.
Use a filter with a lower MERV rating. Indoor air quality won’t be as good, but the AC banging, clanking or thumping at start and stop might cease.
Did you know?
MERV is the industry standard rating. It stands for Minimum Efficiency Reporting Value – which is a way of stating how effective it is in removing small particles from the airflow in your HVAC system. The higher the rating, the more particles and the smaller particles are removed.
3M Filtrete filters use the designation MPR, Microparticle Performance Rating; Honeywell uses FPR, Filter Performance Rating. They are closely equivalent to MERV.
If you’re concerned about indoor air quality and the role your HVAC air filter plays in it, please see our AC/Furnace Filter Guide with comprehensive information about filter ratings and which rating is right for you when trying to remove dust, pollen, dust mites, mold spores, pet dander, smoke, bacteria and viruses from the airflow.
3). The air filter needs to be secured.
If the air filter is clean – or at least not heavily coated in “stuff,” and you want the air filtration it provides, there is a simple fix.
Use tape to hold the filter in place, preventing it from banging and clanging against the frame or grille.
It works! Here are comments from a few homeowners:
“Tape your new filter INTO PLACE!! DUCT, PACKING, ANY KIND OF HEAVY DUTY TAPE WILL DO THE TRICK. No more slamming and banging each time the unit turns on and off.”
“I used tape to hold the filters in place after cleaning the surface that the tape adheres to. PROBLEM SOLVED!”
Pro Tip: If you’re using a MERV-10 filter or higher, it might be worth consulting an experienced HVAC technician about whether the filter is the right choice. Why?
A highly restrictive filter is like a really dirty filter. It might reduce efficiency of the system and make it work harder, potentially causing repair problems or requiring early replacement.
What about an air purifier?
If the technician advises that your system isn’t designed to handle restrictive airflow, the answer might be a free-standing air purifier with the right filter to remove whatever you’re concerned about: Dust, dander, mold spores, smoke, odors, viruses and more.
Indoor Noise with Repair Required
Do you ever overload the clothes washer? If you have a top-load model, which are quickly going the way of the dinosaur, here’s what can happen – When a spin cycle starts or slows to a stop, the weight of the clothes in the tub can cause the tub to bang against the cabinet of the clothes washer. The banging quits as the tub gets up to full-speed spinning, but it can return when the motor stops, and the tub full of wet clothes slows down.
Something similar might be going on with the blower motor and fan in your furnace or air handler.
If a fan mounting is loose or broken, or if the fan is out of balance, the fan wheel might bang against its housing when moving slowly – at startup or slow-down.
When running at full speed, it might or might not bang.
Solution: If you’re handy, you can try tightening the bolts holding the blower motor in place.
If that does not solve the issue, then an HVAC technician should be able to diagnose the issue and recommend the best solution.
Replacing a blower motor costs between $500 and $800. Two of our readers paid $550 and $575, according to our AC & Heat Pump Common Problems and Repair Cost List.
Balancing the blower motor and fan should cost no more than the minimum fee for a service call: $75 - $200.
Two "No Problem" Noises
A loud AC noise at start or stop can also mean:
Neither is a repair issue, but both can be annoying.
It’s possible that an HVAC technician might be able to add ductwork insulation that would reduce either noise.
Outdoor Noise that’s Normal
ACs do make noise, and some are what you should expect.
The most common outside is a scroll compressor. These compressors spin, and when they slow down and come to a stop, it can be with a noticeable “bang.”
Want to be sure? Schedule an HVAC technician to put the AC or heat pump through a multiple-point checklist, clean it and do general maintenance.
The technician will run the AC system, listen for the noise and discuss with you whether it is normal.
Outside Noise that’s Not Normal
There are two parts of the outdoor unit that can bang, clang or thump at the start or end of a cycle – the fan and the compressor.
1). The outdoor fan is broken, the motor is broken, or one of them needs adjustment.
Like the indoor blower motor or fan, an outdoor fan might spin quietly at full speed but wobble and bang when running slower at start or when winding down.
If you’re standing next to the condensing unit – the outdoor unit – when a cycle starts and stops, you will likely be able to determine if this is the issue.
Listen and look – You’ll probably notice if the fan is wobbling, and when accompanied by banging, you’ve almost certainly determined the problem.
Solution: Since the exact issue is hard to nail down, it is probably best to contact an AC repair company to determine the problem and solution.
Here’s the upside – At least you’ll have a good idea of what the problem is. When it comes to working with HVAC repair technicians, letting them know you understand what’s wrong might keep them from attempting to sell you a repair you do not need.
2). The compressor pump support springs are worn out or broken.
Compressors have the potential to make a lot of noise because they work very hard pumping refrigerant between the indoor and outdoor coils.
Many compressors are built with springs to absorb the vibrations of their pumps. If a spring is broken, the pump will wobble in its housing, especially at the start and end of a cycle. This can allow it to bang against the housing.
In the middle of the cycle, when the pump is spinning at full speed, the centrifugal force might cause it to spin upright without banging.
Solution: You have two options – replace the compressor or the entire condensing unit – or live with the noise until the compressor fails.
One experienced AC contractor puts it this way: “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.”
The cost to replace an AC or heat pump compressor ranges from $1,250 to $2,500 if the unit is not under warranty.
Therefore, if the unit is out of warranty, it makes sense to get estimates from replacing the AC or heat pump rather than repairing it. That’s a lot of money to spend on an AC that is older and out of warranty.
Did you know? Even if your AC is under warranty – which is 10 years for most units – you’ll have to pay the cost of labor. Most standard warranties cover parts, but not labor.
If you want to learn more, our HVAC Warranty Guide is comprehensive and insightful.
The Guide will inform you about the manufacturer’s responsibilities, your rights and responsibilities and whether paying for an extended warranty is a good idea.
An Unsolved Mystery?
These indoor and outdoor causes of an AC making loud noise at start or stop cover 90% of all issues.
However, if your problem doesn’t fit these scenarios, we recommend getting an inspection and free estimate for the repair. Using the quick form or convenient phone number for Free Local Quotes is the fastest, most reliable way to get the problem diagnosed with the best options for solving it.