Air Conditioner Noises: What They Mean & How to Fix Them

Modern central AC units or heat pumps typically run at 40–60 dB, about as loud as a quiet conversation​. That’s normal. You might hear a quiet click when it starts or stops. Nothing to worry about that, too.

But when it starts clanking, banging, or screeching? It usually means something’s off. The longer you ignore it, the worse (and more expensive) it gets.

Many Canadian and North American cities have noise bylaws. In Toronto, for example, residential AC units are limited to 50 decibels during the day and 45 dB at night.

In this article, we’ll break down the common air conditioner sounds you might hear, what each one usually means, and what you can do to fix it. Let’s get into it.

Common Air Conditioner Noises & Their Causes

Below, we discuss each noise type in detail with causes, solutions, and tips:

Banging Noise from AC

A loud banging or hammering noise, like something’s slamming inside the unit, usually when the system starts or stops. Sometimes the whole unit shakes with it.

Possible Causes:

• Loose or broken part inside the compressor
• Fan blade or blower wheel has come loose
• Unbalanced blower fan hitting other parts
• Damaged compressor (e.g., broken rod or spring inside)
• Faulty fan motor trying to start
• Wear and tear from switching between heavy summer use and winter shutdown

Solution:

1. Turn off the AC right away.
2. Do a quick visual check. Look at the outdoor unit to see if the fan blade is hitting something or if any panels or screws are loose. If you spot anything obvious, tighten it.
3. If nothing looks off or if the noise is still there, it’s time to call a technician.
4. The technician will open up the unit and check inside. If the compressor has internal damage, it might need a full replacement (costs $1,350 to $2,300) since compressors are sealed.
5. If the compressor is badly damaged and your unit is older, the technician might recommend replacing the entire AC unit instead of sinking money into a costly repair.

Hissing or Whistling Sound from the AC

Hissing sounds like air leaking from a tire or a snake hiss, steady or on and off. Whistling is more of a high-pitched whoosh, like air blowing through a tiny crack.

Possible Causes:

• Refrigerant leak from a crack or hole in the coil or line
• High pressure inside the compressor (can cause a shrieking hiss)
• Leaky internal AC valve
• Air escaping through duct leaks
• Dirty or clogged air filter causing airflow restriction
• Undersized or loose air filter getting sucked into return grille

Solution:

1. If the hissing turned into a high-pitched screech, it might mean the system’s pressure got too high. Keep the unit off until a pro inspects it.
2. For whistling, start by checking your air filter. If it’s dirty, replace it with a clean one of the right size.
3. Inspect visible ductwork and vent joints. Use foil tape or duct mastic to seal any small gaps.
4. Do not try to check refrigerant lines yourself. Refrigerants can be harmful; only licensed HVAC techs can legally handle them in Canada.
5. Call a certified technician. They’ll use tools to find the leak, repair or replace the damaged section, and recharge the refrigerant.

Buzzing or Vibrating Sound from AC

This sound is like a swarm of bees or a loud electric hum. Sometimes you’ll feel it shake the floor or hear stuff in the house rattle. It might come from the outdoor unit (the condenser) or from inside your home.

Possible Causes:

• Loose panel or fan blade vibrating
• Debris stuck in the outdoor fan or blower
• Failing contactor (relay switch buzzing while trying to engage)
• Weak or failing capacitor
• Compressor malfunction or electrical voltage issue
• Frozen evaporator coil making a faint buzzing/hissing
• Window or portable unit not seated properly, causing vibrations

Solution:

1. Turn off the thermostat and the breaker before checking anything.
2. Clear the outdoor unit of sticks, leaves, or any debris caught in the fan or grates.
3. Tighten all the screws on the condenser’s panels that are loose.
4. Try gently wiggling the fan blade. If it moves or wobbles, it could be loose and might need adjusting or replacing by a technician.
5. If you can’t find an obvious cause or if the buzzing is coming from inside the unit, it’s time to call a pro.
6. If the capacitor is faulty, they’ll swap it out. (costs between $150 and $400).

Clicking Noise When AC Starts or Stops

You hear a sharp click or ticking sound when your AC kicks on or off. Sometimes it’s just one click. Other times it might be a fast, repeated clicking that keeps going.

Possible Causes:

• Normal single click from thermostat or relay at startup/shutdown
• Faulty contactor rapidly clicking in the outdoor unit
• Bad relay or control board trying (and failing) to turn the system on
• Malfunctioning thermostat (low battery or internal fault)
• Weak capacitor or stuck compressor (clicks but doesn’t start)
• Metal ducts expanding/contracting with temperature changes

Solution:

1. If clicking keeps happening or the AC won’t start, note where the sound is (thermostat, furnace, or outside).
2. Replace batteries in battery-powered thermostats—weak ones can cause clicking.
3. Don’t keep flipping the breaker or thermostat; it can damage the system.
4. Call a technician to check the contactor, capacitor, thermostat, or control board.

Rattling Sound from AC

Sounds like something is shaking or bouncing around. Kind of like a box of screws or a marble in a tin can. Usually louder when your AC starts or stops, but it can keep going while it’s running.

Possible Causes:

• Loose screws, panels, or fan blade on the outdoor unit
• Debris like pebbles, twigs, or leaves inside the condenser
• Loose or damaged blower fan or housing in the indoor unit
• Vibrating refrigerant lines or motor mounts
• Rattling metal ductwork or loose vent covers

Solution:

1. Turn off the power to your AC at the thermostat and breaker.
2. Tighten any loose screws on the casing or fan guard on the outdoor unit.
3. Remove the top grille and check for debris like sticks, leaves, or stones.
4. Inspect the fan blades. Ensure nothing is stuck to them and they aren’t cracked or loose.
5. Check your indoor unit or furnace. Make sure all access panels are screwed in properly.
6. Peek inside the blower area with a flashlight (power off) to see if anything is loose or caught, like insulation, a broken filter, or a wire.
7. Turn the system back on and listen. If the rattle is gone, great. If not, you’ll need a pro.

Squealing or Screeching Noise from the AC

It sounds like metal scraping or a belt slipping. It might show up when the AC starts, or it could keep going the whole time it’s running.

Possible Causes:

• Worn or loose blower belt (in older systems)
• Bad motor bearings in blower or fan motor
• Lack of lubrication on moving parts
• High pressure inside the compressor (safety valve screaming)
• Overcharged system or failing compressor valve causing pressure buildup

Solution:

1. If the screeching is loud and seems to come from the outdoor unit, shut off your AC immediately. Call a technician immediately.
2. Once a technician arrives, they will determine if the squeal is from the indoor blower motor, the outdoor fan, or the compressor. They might try lubricating first (if applicable) for a blower motor.

Gurgling or Bubbling Noise from AC

Sounds like water running, bubbling, or sloshing through a pipe. Some people say it reminds them of a coffee pot or a backed-up sink.

Possible Causes:

• Refrigerant equalizing after system shutdown (short, harmless noise)
• Clogged condensate drain line causing air bubbles or water backup
• Refrigerant leak bubbling through a pinhole or crack
• Air trapped in refrigerant lines (usually after recent service)
• Slight gurgle from the expansion valve (shouldn’t be loud)
• Internal water management system in portable/window AC units

Solution:

1. Find the condensate drain (usually a white PVC pipe near the indoor unit).
2. Turn off the AC. Open the clean-out cap and clear gunk with a wire brush.
3. Pour vinegar and water down the line to break up buildup.
4. If the drain runs outside, use a wet/dry vac at the end to suck out clogs.
5. Pour water through the line to make sure it flows properly.
6. If gurgling continues and cooling is weak or coils freeze, turn off the system and call a technician.
7. A pro can check for refrigerant leaks and recharge the system if needed.

Humming Sound from the AC

All AC units hum a little, but if it’s getting louder or more noticeable than usual, that could mean something’s starting to go wrong.

Possible Causes:

• Normal sound of fan motor or compressor running
• Fan or blower motor running dry (needs lubrication)
• Bent coil fins or parts touching moving components
• Aging contactor or transformer making electrical hum
• Stalled fan motor or failing capacitor (if unit hums but doesn’t run)
• Misaligned or vibrating metal parts rubbing against each other

Solution:

1. If the humming sound is new or growing, it’s worth checking.
2. If you hear a hum but nothing moves (fan, compressor in the outdoor unit).
3. A humming outdoor unit with no fan might mean a bad capacitor or stuck motor—this needs a pro to fix.
4. For indoor humming, check if the blower motor is running properly. If not, it could be a similar motor or capacitor issue.
5. Check for loose sheet metal or conduit touching the unit. A small piece of foam or weather stripping can help silence metal-on-metal vibration.
6. During your next tune-up, ask the technician to check motor bearings and add lubrication if your unit has oil ports (many older motors do).

Thumping or Clunking Noise from the AC

Sounds like someone’s hitting the unit with a soft hammer or like a shoe tumbling in a dryer. You might hear it once, usually when the AC starts or stops, or it could be a steady thump-thump while it runs.

Possible Causes:

• Unbalanced fan blade in outdoor condenser
• Dirty or bent blower wheel (indoor blower fan)
• Loose motor mounts causing the fan or motor to knock
• Broken compressor mounting spring causing internal “clunk”
• Objects or debris stuck inside the outdoor unit (branch, ice, etc.)
• Metal ducts flexing or popping with pressure changes

Solution:

1. Figure out if the noise is from inside or outside.
2. Turn off the AC and shut off the power at the breaker.
3. Outside: Spin the fan by hand. If it wobbles or hits the casing, that’s the issue. Tighten or gently adjust it.
4. Clear out any debris like sticks, ice, or nests.
5. Inside: If you’re comfortable, remove the furnace panel. Check if the blower wheel wobbles, is dirty, or has a missing blade.
6. If anything seems loose or off-balance, stop and call a technician—it likely needs repair or replacement.

Preventive Maintenance to Reduce AC Noises

The best way to deal with strange AC noises is to prevent them from happening in the first place. Here are some preventive steps and tips:

• Schedule a yearly maintenance and inspection. Spring is a good time in most parts of Canada, before the summer heat kicks in. A technician will check moving parts, tighten loose components, and spot wear before it turns into noise.
• Keep the outdoor unit clear. Make sure there’s at least 2 feet of open space around the condenser. Trim back plants, remove leaves, and keep the area clean to avoid debris entering the fan.
• Change your air filter regularly. Every 3 months is the bare minimum, but if you’ve got pets or allergies, do it more often. A clogged filter strains the system, leading to humming, whistling, or rattling.
• Listen closely. If you start hearing a new sound—like a rattle, buzz, or squeal—don’t wait. Catching the problem early usually means an easier (and cheaper) fix.
• Protect your AC in the off-season. After the cooling season ends, shut off the power to the outdoor unit and consider covering it with a breathable AC cover to keep out debris and ice.
• Consider noise-reduction upgrades. If your unit is older, it might just be loud by design. Some models support compressor sound blankets that can be added to help reduce operating noise.

Keep Your AC Quiet and Reliable

Strange noises are your AC’s way of asking for help. Don’t wait for a minor sound to turn into a major repair.

FurnacePrices.ca connects you with Canada’s top 1% vetted local HVAC companies. Get free quotes, expert service, and peace of mind before the noise becomes a bigger problem.

Looking for a quieter AC? Check out our guide to the quietest central air conditioners to find low-noise models that keep the peace.

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