It’s pretty unsettling to hear your air conditioner making noise even when it’s turned off. You might be wondering, “Is something still running?” or worse, “Is something broken?”
While it’s normal for some AC units to make a bit of noise after shutting down, ongoing or unusual sounds could point to a deeper issue. From lingering fan movement to electrical glitches or even critters making themselves at home, there are several reasons why your system might still be making noise when it’s supposed to be off.
The good news is that most of these issues can be diagnosed and fixed with a little help. In this blog, we’ll walk you through the most common causes and help you figure out what’s normal, what’s not and when it’s time to call in a professional.
Residual Fan or Compressor Noise
If your air conditioner is making noise even after it’s switched off, it might be due to residual fan or compressor activity. This can happen for a few different reasons, and not all of them are cause for concern.
Here’s what to know:
- Fan spin-down: The fan doesn’t always stop immediately. It might take a few seconds (or even minutes) to completely wind down, which can cause a soft humming or whooshing sound.
- Compressor cool-down: Some units run the compressor briefly after shutting off to manage internal pressure or temperature.
- Normal delay cycle: Certain systems include a short delay before turning off completely, especially in high-end or smart models.
- Sign of wear and tear: If the noise sounds rough, grinding, or unusually loud, it could mean parts are worn or loose and need attention.
If the sound is persistent or unfamiliar, it’s worth having a professional take a look.
Expanding or Contracting Ductwork
If you’re hearing popping, banging, or creaking sounds after your air conditioner turns off, your ductwork could be the reason. As the temperature inside the ducts changes, the metal expands or contracts. This is totally normal − but it can be noisy.
If you have ducted air conditioning, here’s what’s happening and what to watch for:
- Temperature shift: When cold air stops flowing, the duct metal begins to warm back up and shift slightly.
- Popping or creaking: These sounds come from the ducts flexing as they adjust to the temperature change.
- Thin or poorly installed ducts: If your ductwork is made from thin metal or isn’t properly supported, the noise can be louder.
- Sudden shutoff: If the system stops abruptly, pressure changes can add to the noise.
While this kind of noise is usually harmless, excessive or sudden changes might be worth checking out with a heating, ventilation and air conditioning (HVAC) technician, who might recommend thorough duct cleaning.
Electrical Issues or Faulty Relays
If you’re hearing clicking, buzzing or humming sounds from your air conditioner even when it’s off, there might be an electrical issue at play. One common culprit is a faulty relay switch.
This component helps control when your AC turns on and off, and if it’s stuck or malfunctioning, it can cause strange noises − even when the system isn’t running.
Another possible issue is a failing capacitor or contactor, which can also emit electrical noises. These sounds might seem minor, but they can signal bigger problems down the track, like short circuits or total system failure.
Here’s what to watch (or listen) for:
- Repeated clicking or ticking sounds.
- A low buzzing or humming coming from the outdoor unit.
- The system trying to turn on by itself.
If you notice any of these, it’s best to turn off the power and call a licensed HVAC technician.
Debris in the System or Outdoor Unit
If your air conditioner is off but still making strange noises, debris might be the culprit − especially in the outdoor unit.
Over time, leaves, sticks, dirt and other debris can get inside the system and cause rattling, tapping or grinding sounds, even when the fan isn’t actively spinning.
Here’s what to look (and listen) for:
- Rattling or clinking sounds: Loose debris like stones or twigs might be stuck near the fan or in the housing.
- Intermittent tapping: A small object caught in the fan blades can cause tapping as the blades move slightly.
- Wind movement: A breeze might shift debris around inside the unit, causing light but odd noises.
- Blocked drainage: Gurgling or bubbling sounds can come from clogged drain lines filled with dirt or grime.
To avoid damage, turn off the power and have a technician safely inspect and clean the unit.
Malfunctioning Thermostat or Control Board
Sometimes, the source of the noise isn’t the air conditioner itself − it’s the thermostat or control board acting up. These components are like the brain of your AC system, telling it when to turn on and off.
If they malfunction, they might send mixed signals, causing parts of the system to stay active or attempt to start up − even when everything should be off.
You might hear:
- Random clicking or ticking from the wall-mounted thermostat.
- Soft humming or buzzing from the control board.
- The fan briefly trying to spin up, then stopping again.
These noises can be subtle but annoying, and they’re usually a sign that something’s not quite right with your system’s electronics.
Left unchecked, it can lead to bigger issues, like short cycling or inconsistent cooling. If your thermostat or control board seems out of sync, it’s a good idea to have a technician check things over.
Refrigerant Flow or Pressure Changes
Even when your air conditioner is off, you might still hear faint hissing, gurgling, or bubbling noises.
These sounds can come from the refrigerant as it settles and equalises pressure within the system. It’s often normal, but sometimes it could signal a leak or other issue.
Here’s what might be going on:
- Pressure equalisation: After the system shuts down, refrigerant continues to move slightly as the pressure balances out.
- Gurgling sounds: This can happen when liquid refrigerant flows through the lines during pressure changes.
- Hissing noise: A mild hiss may be normal, but a constant or loud hiss could indicate a leak.
- Ongoing noise: If the sounds don’t fade after a few minutes, it might be time for a closer look.
If you’re unsure whether the noise is harmless or a warning sign, it’s best to call an HVAC professional for a quick check.
Animal or Insect Activity
Believe it or not, critters might be the reason your air conditioner is making noise − even when it’s off.
Outdoor units, in particular, can attract small animals and insects looking for shelter or warmth. Rodents, birds or even possums may crawl inside and start nesting. You might hear scratching, scurrying or flapping sounds as they move around.
Insects like wasps or ants can also build nests inside the unit, especially around wiring or in the control panel. Their presence might cause unusual buzzing sounds, and in some cases, even electrical issues if they damage internal parts.
Here’s what to listen for:
- Light tapping or scratching coming from the outdoor unit.
- Buzzing or fluttering noises without the system running.
- Sudden stop-start sounds with no obvious reason.
If you suspect animals or insects are the cause, it’s best to shut off power to the unit and call a professional for safe removal and inspection.
What You Can Do About It
If your air conditioner is making noise when it’s off, don’t panic − there are a few simple steps you can take to figure out what’s going on. Some sounds are harmless, while others may need a professional’s touch.
Here’s what you can do:
- Listen closely: Try to identify the type of sound − buzzing, tapping, hissing or clicking can all point to different issues.
- Inspect the outdoor unit: Look for visible debris, damage, or signs of animal activity.
- Check the thermostat: Make sure it’s working properly and isn’t sending confusing signals to the system.
- Turn off the power: If the noise continues when everything is off, shut off power at the breaker for safety.
- Call a technician: If the noise is persistent, loud, or unusual, a licensed HVAC professional can diagnose the issue and make sure everything’s running safely and efficiently.
Avoid Costly Repairs Down the Track
Hearing your air conditioner make noise when it’s off can be confusing − and a little concerning. But in many cases, it’s something simple like expanding ductwork, leftover fan movement, or pressure changes in the refrigerant lines.
That said, unusual or ongoing sounds could also point to deeper issues like electrical faults, debris in the unit or even animals making themselves at home.
The key is to pay attention to the type of noise and when it happens. If it sounds odd, is happening frequently, or you just want peace of mind, it’s always a smart move to call in a licensed technician for AC maintenance.
A quick check-up can help you avoid costly repairs down the line and keep your system running quietly and smoothly − all year round.
Please note: This information is provided for advice purposes only. Regulations differ from state to state, so please consult your local authorities or an industry professional before proceeding with any work. See our Terms & Conditions here.
FAQs About AC Noises When Switched Off
Do you have any questions about your air conditioner making noises when not in use? If you do, we’ve got the answers right here.
Yes, in many cases, it's completely normal. You might hear the fan spinning down, ductwork creaking as it adjusts to temperature changes, or even a soft gurgling sound from refrigerant pressure balancing out. These are usually harmless. But if the noises are loud, persistent, or new, it’s worth investigating further.
Clicking sounds could be coming from the thermostat, control board, or a stuck relay switch. These components help control when your system turns on and off. If something’s faulty, it might keep trying to send signals, causing repeated clicks. An HVAC technician can quickly test and replace any failing parts if needed.
Absolutely. Small animals like mice, birds, or insects can squeeze into the outdoor unit, especially if it’s not properly covered. They can cause scratching, buzzing or rattling noises. Regular inspections and keeping the area clear can help prevent unwanted guests.
