Hearing loud or strange noises from your outside AC unit can be scary.
Without closely examining the unit, you can’t understand exactly where the sound is coming from, but these explanations of the different sounds and their causes should help to give some indication of the problem, the severity, and how to fix it.
Many sounds can be a sign of something that can cause serious long term damage to your AC system. For this reason, we strongly recommend you turn off the AC and get it inspected by a professional to help prevent any damage.
- Banging when stopping/starting
- Rattling
- Squealing/Screeching/Whistling
- Buzzing/Humming/Vibrating
- Clicking/Popping
Outside AC making a loud noise when starting or turning off
A loud banging or clanking sound that happens exactly when you turn the AC unit off or on usually means the compressors springs are broken or worn.
Broken support springs cause the compressor to lose balance and wobble as it starts up or slows down, causing it to hit into the side of the unit and create a banging sound. Broken springs will need to be replaced by a professional.
Outside AC making loud rattling noise
A loud rattling or banging noise that occurs when the AC unit is running is usually caused by a loose part or foreign object moving about inside the unit.
Cause 1: Loose parts.
Loose parts in the outdoor unit can knock against each other and cause a loud banging or rattling sound. This is often a loose condenser fan banging against the unit, but could easily be a connecting rod or crankshaft. In some cases the loose part will need tightening, but it may need to be replaced entirely.
Cause 2: Trapped debris
Debris such as twigs, leaves or stones can get into the outdoor unit and rattle when the unit is turned on. The debris will need to be removed or it can damage the parts.
Outside AC unit making a squealing high pitched noise
A high pitched noise can be either a whistling noise or a screeching noise. A screeching noise is likely to be the sound of parts grinding together. A high pitched whistling sound is more likely to be caused by high pressure or circulation issues.
Cause 1: High refrigerant pressure
A high pitched whistling sound coming from the AC compressor can be caused by high refrigerant pressure. When this occurs, you may notice your AC unit turning off and on as it detects the pressure and shuts down. If you hear the screeching but the compressor does not shut down, turn off the system immediately.
Cause 2: Loose pulley or belt
Belt driven AC systems can come loose and damaged over time which can cause a screeching sound as the parts rub together. Most newer AC units have direct drive motors instead of belts or pulleys, but they are common in older units. If the belt has become worn over time it can slip off and may need replacing.
Cause 3: Faulty bearings
A screeching sound on your outside AC unit can be caused by failing bearings in an electric motor. If the condenser fan motor or compressor motor is making a screeching sound then the bearings are probably failing and need replacing.
Outside AC making hissing noise
A hissing noise coming from your outdoor AC unit is usually caused by refrigerant leaking. Refrigerant is usually circulated throughout tubes in the house but can escape if there is a small hole in your AC system.
Other signs that you may have a leak are a chemical smell near the hissing sound, air being released and an increase in your energy bill. If you have a leak, it will need to be found, fixed and the refrigerant topped up.
Outside AC making loud buzzing or humming noise
When you hear a loud constant buzzing, humming or vibrating noise coming from an outside AC unit, this usually means you have an electric problem in the unit.
The most common electrical problems which cause buzzing sounds are:
Cause 1: Loose wiring
Old wiring that becomes unsoldered or frayed can cause faults with the electric connection which creates a buzzing noise.
Cause 2: Faulty motor
If the condenser fan motor stops working then you’ll hear a humming noise and the outdoor fan won’t turn. A faulty compressor motor can also create a humming sound.
Cause 3: Faulty capacitor
The capacitor is a small metal object on the side of the outdoor unit. If the capacitor is damaged or rusted it can stop working. This usually prevents the AC unit from turning on and will cause a buzzing sound.
Outside AC unit making clicking or popping noise
If your outside AC unit is making a popping or clicking noise, you should turn off the power right away. There are a number of causes, some more harmful than others.
Cause 1: The unit is freezing
If the temperature is too low, this can cause ice to form. The popping sound comes from the ice cracking and moving inside the AC unit. There may be an underlying problem causing this such as poor drainage. Many AC units contain a sensor which can detect ice and automatically enter thaw mode.
Cause 2: Water inside the unit
If water enters the AC unit it can make a popping sound as the unit tries to remove the water. This can cause serious damage to the internal components and is most common after a rainstorm.
Cause 3: Liquid refrigerant in the compressor
A popping sound can also be caused by the compressor accidentally sucking up liquid refrigerant. This can cause serious damage to your compressor.