AC Making Noise When It’s Off? Here Are the Causes

Is your AC making noise when it’s off? That’s not just an oddity; it might be signaling a problem with your air conditioner.

In this article, I’ll dive into the potential causes of noises that an air conditioner makes while it’s off. I’ll also provide fixes for the noise issues and offer solutions to restore the silence!

The Types of Noises That an AC Makes While It’s Off

Humming or Buzzing Noises

  • Transformer: A faulty or aging transformer could emit a consistent humming or buzzing noise, often due to the vibration of its laminated steel core or windings.
  • Contactor: A contactor might make a buzzing sound while off due to vibrations in its electromagnet. Pitted contacts on an old contactor may arc, creating a buzzing noise.
  • Capacitor: A capacitor can hum, particularly if it’s starting to fail.
  • Compressor: A compressor will sometimes buzz if it fails to start up.
  • Condenser fan: A condenser fan will hum or buzz if it fails to start spinning.

Hissing or Squealing Noises

  • Compressor: A compressor with a malfunctioning pressure relief device or safety valve could make a hissing or squealing noise.
  • Thermostatic Expansion Valve (TXV): A malfunctioning TXV might emit a hissing or gurgling sound if the refrigerant is boiling off inside the valve.
  • Refrigerant Leak: A leak in your air conditioner’s refrigerant system may cause a hissing noise as refrigerant leaks out.

Rattling or Ticking Noises

  • Loose or Damaged Parts: Loose parts could rattle from wind blowing.
  • Air Filters: Poorly fitting air filters could rattle in their housing due to air drafts in your home.
  • Fan Motor: A fan motor with worn-out bearings might emit a ticking sound as the rotor hits the stator.
  • Thermal Expansion: Some metallic components in your AC may produce a ticking sound as they heat up or cool down.
  • Ductwork: The ductwork in your home may produce ticking or banging noises when it expands and contracts.

Gurgling Noises

Chattering or Fluttering Noises

  • Compressor: Faulty internal valves within the compressor could flutter or chatter.
  • Contactor: A contactor with a weak coil or loose connection might cause a chattering sound.

Loose or Damaged Parts

Air conditioners, while off, can make noise due to the movement of loose or damaged components. Specifically, elements like the blower wheel, fan blades, or the condenser coil can move slightly, due to gravity or airflow from the environment, which can cause rattling or clanging sounds.

  • How to fix: Regularly inspect the unit for any loose components and tighten them using the appropriate tools. If you find damaged parts, like fan blades, you should replace them with new ones according to the manufacturer’s instructions.

Air Filter

Next up are air filters. Air filters can cause noise in two ways.

Firstly, when the filter is loose in its slot, the filter may rattle or make a flapping noise as air drafts move it around when your AC is off.

Secondly, the debris in the filter can sometimes dislodge and enter the ductwork, where it can move and create noise due to air currents in the home.

  • How to fix: Regularly inspect and clean the air filters. If the filter is disposable, replace it as needed. Ensure that you are using the correct filter size, and if your filter is loose in its slot make modifications to the filter slot to provide a snug fit.

Refrigerant Leak

A leak in your AC’s refrigerant system can cause a hissing or bubbling sound, even when your air conditioner is not running. This happens when refrigerant leaks out of a component in your AC.

  • How to fix: Contact an HVAC professional – they will use a refrigerant leak detector to locate any potential leaks in the system. Then they will repair the leak by brazing and recharge the system with the correct amount of refrigerant.

Thermal Expansion and Contraction

Thermal expansion is a phenomenon that causes the metallic materials in your air conditioner to expand and contract, producing a ticking or cracking sound.

This can occur in the compressor, evaporator coil, and condenser coil in the outdoor condensing unit, where heat is released.

  • How to fix: There is no quick fix for thermal expansion since it is a normal process of heat exchange. Most new AC units stop making thermal expansion noise after a few weeks of use.

Condensate Pump

Condensate pump makes noise in air conditioning system

The condensate pump, responsible for removing the condensation produced by your AC, can create noise even when the unit is off.

After your AC unit has been running, there can still be residual water that needs to be pumped out. If the pump is malfunctioning or if the drain line is blocked, it could cause a continuous or intermittent noise.

  • How to fix: Inspect the condensate pump and drain line for any clogs and clear them. If the pump is not working correctly, you might need to replace it.

Thermostatic Expansion Valve (TXV)

The TXV, which controls the flow of refrigerant into the evaporator coil, can make a hissing or gurgling noise if it malfunctions or if the refrigerant is boiling off inside the valve even after the system is shut off.

If the valve is malfunctioning or if the system is shut off suddenly, it could create a hissing or gurgling sound due to the abrupt stop of refrigerant flow.

The thermostatic expansion valve is located at the indoor furnace or air handler, so if you hear noise coming from the unit inside your home, attic, or basement, it could be the TXV.

  • How to fix: Thermostatic Expansion Valves are part of the air conditioner’s refrigerant system, so it’s best to call an HVAC professional to have them confirm the superheat settings on the TXV and ensure it’s working correctly.


The ductwork in your home can create noise due to thermal expansion.

After the air conditioner is turned off, the ductwork will start to heat up. As it does, the metal can expand, causing popping or pinging noises. This is more common in older ductwork.

  • How to fix: If you find issues in the ductwork like loose joints, secure them with appropriate hardware. In severe cases, consider replacing sections of ductwork.


bad ac capacitor swollen top

The capacitor, which stores and supplies power to the compressor and fan motors during startup, could make noise when it’s malfunctioning.

Even when the unit is off, a capacitor can discharge, creating a popping sound. If the capacitor is leaking oil or bulging, it can create a humming or buzzing sound.

  • How to fix: Use a multimeter to test the capacitor and compare it to the rating on the capacitor. If the capacitor is swollen, leaking, or bulging, it is faulty – replace it with one of the same rating.


Step down transformer in air conditioner

The transformer in your air conditioning system steps down the voltage from the main power supply to a lower voltage for your air conditioner’s control system to use.

If the transformer is aging or malfunctioning, it may create a constant humming or buzzing sound, even when the system is off. This is due to the vibration of the laminated steel core or the windings due to a phenomenon called magnetostriction.

  • How to fix: If the transformer is buzzing excessively, check the mounting hardware and tighten it if loose. If this doesn’t help, replace the transformer.

Condenser Fan Motor

Rattling noise from AC condenser fan shroud

The condenser fan motor in your AC unit outside could potentially make noise even when the air conditioner is off. This could be due to the motor being burnt out, bad bearings, or a bad capacitor causing the motor to stall in place.

Another cause of condenser fan noise is wind. Wind can cause the condenser fan blade to spin when the unit is off. I’ve seen this happen when a fan shroud came loose, and the spinning condenser fan blade caused a rattling noise from the fan shroud.

  • How to fix: Turn off the power to the AC and manually spin the fan blade. If it grinds, you’ll need to replace the motor. If not, test the AC unit’s capacitor.


AC contactor buzzing

The contactor, which controls the flow of electricity to the compressor and condenser fan motor, might make a buzzing noise due to residual magnetic fields causing vibrations even when the AC is off.

The electrical contacts in the contactor can also make noise. If the contacts are worn, pitted, or burned, they can cause arcing, which makes a buzzing or zapping sound.

  • How to fix: If the contactor is making a buzzing sound, you can tap on it (carefully with an insulated tool) to confirm if it’s the contactor – this might make it stop buzzing. Otherwise, it might be bad and need replacement. Ensure that the replacement meets the voltage rating, amperage rating, and number of poles to maintain system compatibility.


The compressor, a significant component of your AC system, could potentially make noise when the unit is off for a few reasons.

Firstly, internal valves within the compressor might be faulty or worn out, causing them to flutter or chatter as they fail to close properly.

Secondly, refrigerant lines connected to the compressor could vibrate due to pressure changes, especially if the compressor shuts off suddenly.

  • How to fix: A noisy compressor may be due to internal issues which often require replacing the entire compressor, so you’ll need to call an HVAC professional to assist.

What’s Next?

When dealing with noises coming from your AC when it’s off, these are just possible sources.

Regular inspection and maintenance are vital in preventing these issues from escalating and ensuring a longer lifespan for your air conditioning unit.

These noises can be symptoms of more significant issues, so it’s best to consult a professional HVAC technician if you’re hearing anything out of the ordinary.

Hi, my name is Trey Lewis and I’m the founder and chief editor at HVAC Training Shop. My goal for this website is to help homeowners troubleshoot and maintain their home’s HVAC systems. Whether it’s changing an air filter, troubleshooting a blower motor, or just buying a new humidifier, I want to make sure that you’re covered.

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