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Why Is My Outside Air Conditioning Unit Making Loud Noises?

You rely on your AC unit to keep your home cool in the hottest parts of summer. But it can be really annoying if your outside air conditioning unit keeps making a loud noise whenever you use it.

Often, these noises are signs of specific problems within your air conditioner. They can indicate that you might need everything from a simple tune-up to a completely new HVAC system.

Keep reading to learn more about the types of noises your outside AC unit may be making and what to do about them.

Screeching and squealing noises

This sound is sort of like the sound that’s made when metal grinds against metal. If that’s what you’re hearing, then chances are you have one of the two following issues.

  • First, you may have a malfunctioning condenser fan motor. This helps your compressor pump liquid refrigerant throughout your entire air conditioning system.
  • Second, you may have a broken compressor. This is the key part of your air conditioner, which allows it to circulate refrigerant throughout your house. When the motor inside of the compressor begins to fail, it can cause this kind of screeching sound. This can be a costly issue to fix, unfortunately.
Sometimes this problem will manifest as a high-pitched squeal. That’s a sign that your air conditioner has a lot of built-up pressure within it. This is dangerous and can potentially cause an explosion. So you’ll want to turn off your system and get an HVAC professional out to your home as soon as possible if that’s what you’re hearing.

Banging and slamming

You may also hear something that sounds like banging and slamming coming from your outdoor unit. In that situation, you’ve likely got one of three issues.

  • First, your compressor may have broken internal support springs. These balance the compressor so that it can operate at its full potential without moving around. When they go out, the compressor can hit nearby components. The only way to fix this is by replacing the compressor entirely because the parts are bundled together. But this problem isn’t something that you necessarily need to deal with right away. You can just live with the sound for a while if you want, since replacing a compressor can be really expensive.
  • It’s also possible that your condenser fan is knocking up against the air conditioner. You can fix this by either tightening a few loose bolts or replacing your fan assembly with a newer one.
  • Finally, these sounds can also be indicative of a broken compressor. This may be the issue that you’re having if you’ve checked the other two potential problems and haven’t been able to get rid of the noises.

Humming sounds

If you’ve got an outside air conditioning unit making a loud humming noise, then there are three potential problems you may be experiencing.

  • First, you might need to lubricate your condenser fan motor. If this is your problem, the sound may begin as a humming but turn into a grinding over time. You can lube the fan motor yourself or call out a professional to help.
  • You may also have a contractor relay switch issue. This is a low-voltage switch that’s inside your outdoor unit. Its purpose is to turn the unit on when the temperature rises enough to require cooling. When this part isn’t working properly, you may hear something like a “chattering noise” coming from the unit. It can turn into a louder buzzing sound if left unchecked. You should expect to pay between $100 and $400 to fix this.
  • Finally, you could have bent fin coils. This can happen from pressure washing, tree debris, and various types of natural wear and tear. You can usually fix the problem by using a simple metal comb to straighten any bent coils that you find.
  • If you experience a humming noise when you try to start your AC, it could indicate you might have a dead capacitor.

Buzzing noises

Buzzing noises are another common problem in outdoor units. They’re usually indicative of a bad fan capacitor.

Your fan capacitor is like a big battery that helps to turn the AC motors on and off. Over time, it will weaken, like any other battery, and lose some of its ability to hold an electrical charge.

With a bad capacitor, your air conditioner can take in more power than it needs, which is what leads to the buzzing noise. You can usually look for visual cues to see if you’ve got a bad capacitor, including:

  • A top that looks swollen
  • Rust or corrosion near the bottom of the capacitor
  • Leaking fluid

The good news is that replacing a capacitor should only cost $60 at most. But that’s if you do it yourself, and most people shouldn’t because it’s a bit dangerous to work with. Still, you can probably get an HVAC professional to replace yours for under $200.

Other reasons for a buzzing noise

Although a faulty fan capacitor is the main reason why you may hear a buzzing noise, it’s not the only one. Here are a few more.

  • You might have a circuit breaker that’s failing to trip. This is dangerous, as the problem will allow more electricity to surge through your home’s wiring than it’s rated to receive.
  • It’s also possible you’ve got a loosely connected wire. You may even see some sparks near the wiring if it’s not connected all of the way.
  • An issue with your contactor relay switch can also cause a buzzing noise, which may begin as a hum.
  • Finally, the problem could also lie within a faulty motor. Various obstructions near your copper lines can cause them to rub against one another and create a buzzing sound.

These are all things that you can investigate on your own with further research. Or you can ask an HVAC professional to help you out with them.

Popping sounds

Sometimes you’ll hear popping noises coming from your outdoor AC. If that’s what you’re hearing, then you’ll want to turn off the system immediately.

Popping noises are a sign that your compressor is sucking up liquid refrigerant instead of gas.

With a normal AC unit, the refrigerant will absorb warm air from your home and turn it into a vaporous gas. It’s this gas that is supposed to enter into the compressor in your outdoor unit.

But sometimes, there can be a problem with an AC unit that causes the liquid refrigerant to do this instead of gas. That can completely destroy your compressor, which is really expensive to replace.

So the best solution to this problem is to turn off your HVAC system and reach out to a professional who can help.

Hissing noises

You may also have an outside air conditioning unit making a loud hissing noise. If that happens, it’s typically a sign that your unit is leaking refrigerant.

Refrigerant gets circulated through copper lines that create a closed loop. This means that you should never run out of refrigerant under normal circumstances.

However, sometimes that closed loop can begin leaking. This is what causes the hissing noise, as the impact of a leak creates a change of pressure, sort of like air when it escapes from a balloon.

There really isn’t a way to fix this on your own. The best option is to call a professional who can do it for you.

You may also want to look for some of the other major signs of a refrigerant leak to confirm this, such as:

  • Unexplained higher energy bills
  • Warm air coming out of your vents when it should be cool or neutral
  • A chemical odor coming from near the refrigerant lines
  • Ice on the ground near your outdoor refrigerant lines

A hissing noise can also be caused by a compressor valve leak. This controls pressurization within the refrigerant line. You’ll definitely want to call an HVAC tech for this problem as well.

Finally, your hissing noise could also be coming from high levels of pressure within the compressor. This issue often leads to shrieking noises after some time has passed. The solution is to shut down your unit and call a professional.

A rhythmic thumping

You may also hear something like a rhythmic thumping coming from your outdoor air conditioning unit. When that happens, it’s often a fan blade or coil that has gotten loose. You typically just need to tighten the screws on this part to fix the issue.

It’s also possible that you’ve got a plastic base supporting your AC unit. In that situation, you may want to replace the plastic with a wooden base, which can minimize any unwanted vibrations.

It’s also worth mentioning that you can get a rhythmic thumping when your refrigerant line touches the wall. Adding some insulation or moving the line away from the wall can solve the problem.

Cracking noises

Sometimes your air conditioner’s evaporator coil may freeze, which can lead to a cracking noise coming from the unit. This may occur when your fins are dirty, the temperature is too low, or you’re having issues with moisture draining.

Basically, all of these problems can cause the evaporator to freeze. And when that happens, ice can fall from it, which may be the cracking noise that you’re hearing.

If you want to try solving this problem on your own, you can take apart your AC system and wash the fins with some dish soap. But if you haven’t done this before, it’ll likely be much easier just to let a professional technician do it for you.

Rattling sounds

You may also hear your air conditioner making a rattling noise as well. This can happen for a few reasons.

For starters, you may have loose sticks, dirt, or leaves within the air conditioner’s condenser unit. The best way to solve this problem is to take the unit apart and get rid of any items that shouldn’t be in there.

It’s also possible that the rattling you’re hearing is the result of loose screws. These can exist in both your indoor and outdoor units, so it’s worth checking both. Fixing a loose screw is pretty easy. You just need to locate it and use a screwdriver to tighten it.

Finally, the absolute worst-case scenario is that your AC could have a faulty electrical contractor. You’ll want to hire a professional to fix this issue as quickly as possible because it could damage your compressor severely if left unchecked.

What sounds are normal for an outdoor unit?

Lots of people worry about an outside air conditioning unit that’s making loud noises. But it’s worth noting that some of these noises are perfectly normal. We’ll cover some of those below.

  • For example, you might hear a constant dull humming. This is totally normal, and many AC systems will emit this noise during the course of standard operations. But if you ever notice the humming change its pitch or become louder for seemingly no reason, then that could be a sign that something more serious is afoot.
  • It can also be completely normal to hear a small squeaking or squealing when your AC system is first starting up. These shouldn’t remain while the system is in operation. If your unit has been on for 10-15 minutes and you still hear them, that’s a sign that you’ve got a more serious problem.
  • Some air conditioners will also make slow pulsating noises. These can be just normal signs of standard operation. But if they’re really loud and somewhat rhythmic, then it can be a sign of a problem.
  • Finally, you may get a buzzing noise just as the AC turns on, which stops when the unit is fully running. This is totally normal. As long as the buzzing or humming that you’re hearing isn’t super loud, it shouldn’t be anything to worry about.
Michael Joseph