HVAC Training Shop is reader-supported. As an Amazon Associate, I earn from qualifying purchases.
Is your outside AC unit making a strange noise? That’s a cause for concern.
Fortunately, you can get a good idea of what kind of problem your AC has based on the type of sound that it’s making.
In this article, I’ll go over what you should do if your outside AC unit is making a noise.
I’ll also go over all of the different types of sounds that an AC unit makes—so you can figure out what’s wrong with your AC no matter what type of noise it’s making.
Outside AC is making a loud noise and not cooling
If your outside AC unit is making a loud noise and not cooling, then it likely has a compressor, condenser fan, capacitor, or contactor issue.
In order to properly diagnose the issue, you’ll need to pinpoint the origin and type of sound.
For example, if you hear a grinding noise coming from the inside of the AC unit, then it’s likely the compressor that’s gone bad.
Or if you hear a buzzing noise coming from the AC’s electrical panel, then it’s likely the contactor that’s not working.
In any case, I recommend going through my list of the different types of noises that an AC unit makes to diagnose the issue.
Outside AC is making a loud noise and the fan is not spinning
If your outside AC unit is making a loud noise and its fan is not spinning, there’s a good chance that the condenser fan motor is bad or it has a bad capacitor.
I recommend testing the condenser fan capacitor first to see if it’s bad.
Most AC units have a dual-run capacitor—two capacitors in a single component.
You’ll need to remove the wires from the dual run capacitor and test the capacitance between the fan and common terminals.
If the capacitor’s farad rating is out of range of its specification, the capacitor needs to be replaced.
If the capacitor isn’t the issue, then the fan motor may need to be replaced.
If you’re unsure, I recommend getting an HVAC professional to check out your AC unit.
Outside AC is making a buzzing noise every few minutes
If your outside AC unit is making a buzzing noise every few minutes, then it’s most likely the compressor overheating.
When a compressor overheats, its thermal overload trips to prevent the compressor from running.
After the compressor cools off, the thermal overload resets, then the compressor tries to start again.
The buzzing that you hear every few minutes is the compressor trying to start when the thermal overload resets. When the compressor fails to start, the thermal overload trips.
So why doesn’t the compressor start up?
It could be due to one of these things:
- Bad capacitor (most common)
- Refrigerant leak
- Loose electrical connection
- Bad electrical supply
- Bad windings in the compressor
I recommend testing the compressor’s capacitor first—that’s the most common culprit. If the capacitor is bad, swap it out for a new one and your unit should start cooling your home again.
If the capacitor isn’t the problem, then it could be any of the other things I mentioned above.
At that point, I recommend calling an HVAC professional to find the issue because it could be a difficult one to diagnose and repair.
Outside AC is making a loud noise when turning on
An outside AC unit that makes a loud noise when turning on is usually not a cause for concern.
Most AC units are quite loud when they turn on—even if there is nothing wrong with it.
If your AC unit has a scroll compressor, it will be even louder than other units.
Scroll compressors are notoriously loud, especially when starting up.
That doesn’t mean that there’s anything wrong with it, scroll compressors are just plain loud.
If your AC continues to make a loud noise while it runs, then that’s a cause for concern.
Check out my list of the types of AC unit noises to figure out what’s causing the issue.
List of the different types of noises that an outside AC unit makes
The noise that your AC unit is making gives you a good indication of what the problem is.
Here’s a list of all the different types of noises that an AC unit makes:
- Banging noise
- Grinding noise
- Buzzing noise
- Clicking, clanking, or rattling noise
- Screeching or squealing noise
- Hissing noise
I’ll go over them below.
Banging noise
A banging noise from your AC indicates a large loose component such as the compressor or condenser fan.
If you hear a banging noise from your AC, you’ll need to figure out where the sound is coming from before addressing the problem.
If the compressor is making a banging noise, then its bolts may have come loose from the bottom of the unit.
A compressor may also make a banging noise if its isolation foot is damaged or worn out. The isolation foot is a piece of rubber that sits between the compressor and the bottom of the AC unit.
So if the isolation foot gets worn out, the compressor may make a noise as it makes contact with the bottom of the unit.
Another cause of a banging noise from an AC unit is the condenser fan. If the condenser fan blade breaks or becomes unbalanced, then the condenser fan will make a banging noise when it runs.
If your condenser fan blade is unbalanced or broken, it will need to be repaired or replaced.
Grinding noise
A loud grinding noise from your AC unit is almost always the compressor.
If you’re hearing a loud grinding noise from your AC, you should call an HVAC professional immediately.
A grinding compressor is a time bomb—there’s no telling how much time it has left before it stops working and you lose cooling.
Replacing an AC compressor is an expensive job—most people opt to get a whole new unit if their compressor isn’t under warranty.
Buzzing noise
Buzzing noises from an outside AC unit occur for a variety of reasons:
- Bad contactor
- Bad compressor
- Compressor isolation foot damaged
- Bad Capacitor
- Bad reversing valve (if your unit is a heat pump)
A bad contactor is one of the most common causes of buzzing from an AC unit.
If you hear a buzzing noise coming from your AC’s contactor, turn off your AC unit and tap on the contactor with the handle of an insulated screwdriver. This sometimes gets rid of the buzzing noise.
If the contactor is still buzzing, then it likely has corroded contacts and needs to be replaced.
Clicking, clanking, or rattling noise
Clicking, clanking, and rattling noises from your outside AC unit usually indicate a loose part.
Here are some common causes of clicking, clanking, and rattling noises:
- Loose condenser fan blade
- Cracked or broken condenser fan blade
- Loose panel on the AC unit
- Loose fan shroud (the metal grate on top of the unit)
- Bad isolation foot under AC unit
- Line set rubbing against the house
If a screw on the outside of the AC unit is loose, a panel may start clanking around from the vibrations of the AC unit when it’s running.
If you hear a clicking, clanking, or rattling noise coming from your AC unit, you’ll first need to figure out what’s causing the noise. Is it the condenser fan? Or is it something else?
Once you’ve figured out what’s causing the noise, you’ll need to determine if you can fix it yourself or if you’ll need to call a professional.
If it’s something simple like a loose screw on a panel, then you can just shut off the power to your AC unit and tighten up the screw.
If it’s something more severe like a broken condenser fan blade, you may need to call in an HVAC professional.
One thing that I should mention about loose panels is that tightening them up may not fix the noise. Sometimes, you’ll need to slip a shim (such as a small washer or piece of rubber) between the screw head and the panel to stop the rattling noise.
Screeching or squealing noise
A high-pitched screeching or squealing noise from your outside AC unit is an indication that your compressor has high refrigerant pressures.
If you hear a screeching or squealing noise from your AC compressor, you need to get it fixed as soon as possible.
If your AC runs with high refrigerant pressures, your compressor will eventually break down and need to be replaced.
Another cause of a screeching or squealing noise from an outside AC unit is bad bearings in the condenser fan motor.
If you hear a screeching noise from your AC condenser fan motor, its bearings are going bad. You’ll need to replace the motor as soon as possible.
If your condenser fan motor’s bearings are going out, the motor will eventually stop working and your AC won’t cool your home anymore.
One trick is to squirt a bit of lubricating oil down the shaft and into the motor. This usually gets you a few more weeks on the motor until someone is able to replace it.
If you have an older unit, the condenser fan motor might have an oiling port on it. So you can oil up the bearings and get more runtime out of the motor.
This synthetic oil keeps outdoor motors and fans running smoothly, even in low temperatures. It comes with an applicator tube for easy injection into motor oiling ports.
A cracked condenser fan blade can also make a screeching or squealing noise.
A cracked condenser fan blade has the tendency to rub up on itself when the fan spins. This creates an ear-piercing screech when your AC unit runs.
If your condenser fan blade is cracked, the fan blade will need to be replaced.
Hissing noise
A hissing noise from an outside AC unit usually indicates a refrigerant leak.
If you hear a hissing noise coming from your outside AC, try to pinpoint the noise and spray a soapy water solution on the suspected area. If you see bubbles, that confirms there is a refrigerant leak in your AC.
Look for a refrigerant leak in your outside AC unit in these areas:
- Condenser coil
- Refrigerant lines
- Refrigerant line connection points and valves
- Filter drier
A refrigerant leak needs to be repaired by an HVAC professional as quickly as possible.
The HVAC technician will drain the system of refrigerant, repair the leak, then recharge and recommission the system.
An overheated compressor also makes a loud hissing noise.
If your AC unit is running and it suddenly starts making a loud hissing nose, the compressor is overheating.
The hissing noise comes from a pressure relief valve on the compressor.
If you hear a loud hissing noise from your AC unit while it’s running, turn it off immediately.
Next, take a look at the condenser coil on the unit. If the condenser coil is excessively dirty or plugged up, there’s a good chance that’s the issue.
Another cause of an overheating compressor is a broken condenser fan motor.
If the condenser fan motor doesn’t start up when the compressor does, then the compressor will overheat and make a loud hissing noise. The condenser fan motor will need to be repaired before the AC unit can run again.
