Why Is My Car AC Making Hissing Noise?

(Last Updated On: March 14, 2024)

A question that we frequently encounter is, “Why does my car produce a hissing sound?” This could simply be a standard noise from your car’s air conditioning system, or it could mean a critical issue.

We’ve talked to professionals and done an extensive research to bring you the answer to the question. Read on as we discuss the various reasons behind this problem.

So, let’s get to it.

Here's where brings the reasons why your car ac making a hissing noise and how to fix it.
In this article, we’ll bring the information about the hissing noise of AC.

Is car AC making hissing noise normal?

When you hear a hissing noise when you turn on the AC, and the noise lasts just a few seconds, it may not be an AC problem. You hear the sound of the refrigerant as it passes through the expansion valve. Most AC systems cycle output or compressor clutch. As the pressure rises and lowers, the audible action of it metering into the evaporator causes the hissing sound

Car Ac making a hissing noise is a common problem in car
If the AC makes a hissing sound whenever it’s on, it can signify a refrigerant leak.

Another everyday occurrence is the AC compressor hissing after shut-off. This happens when you turn off your vehicle. This action causes the refrigerant to move from the high-pressure side to the lower pressure side. As a result, the pressure equalizes, thereby releasing the hissing sound. 

However, if the AC makes a hissing sound whenever it’s on, it can signify a refrigerant leak. It can also be a leaky internal valve or too high AC compressor pressure. You should investigate this type of hissing to prevent damage. 

While it can be something you can diagnose at home, it’s always advisable to have a professional check it out. 

The causes and how to fix it

The causes of your car AC making a hissing noise
3 causes lead to your car AC making a hissing noise.

1. Refrigerant lines leak 

A hissing sound caused by refrigerant lines leak is heard because of the escaping gas.
A hissing sound caused by refrigerant lines leak is heard because of the escaping gas.
Credit: mechanics.stackexchange.com

A hissing sound caused by refrigerant lines leak is heard because of the escaping gas. These leaks occur along with the rubber parts like hoses, gaskets, and O-rings, or the leak may come from metal parts and connections that develop over time. These leaks can come from the evaporator, condenser, accumulator, or receiver drier. 

When this happens, gaseous refrigerant under high pressure will escape. This is a significant problem. It means that the air conditioner is discharging, putting the compressor and the entire unit in distress. 

Not to mention, inhaling Freon is potentially dangerous to your health. 

How to diagnose refrigerant lines leak

Visual inspection

Here, you’re looking for the residue of refrigerant or PEG oil. The first thing you’ll want to do is check the condenser, refrigerant lines, and compressor. If the leak is slow, you’ll find the leak somewhere along the lines. 

Soap solution

Using a soap solution is one of the most effective methods of discovering refrigerant leaks. All you need is a spray bottle and a thick soap solution. Then, charge your car’s AC system fully and spray the soap solution on all the components. 

Bubbles will form wherever the refrigerant is leaking. This technique is ideal for detecting line leaks except for the evaporator or the compressor front seal. 

Black light 

If your refrigerant has been mixed with a UV dye, the dye will show up under a black light. Again, this method is only effective if the leak is not in the evaporator or front seal. 


If all else fails, you can use a device designed for finding car AC leaks – the sniffer. You’ll need to dust the car and wipe the exterior of the AC components. Then conduct the test in a closed area to prevent air and dirt from interfering with the results. 

How to fix refrigerant lines leak

Evaporator fix

If you discover that the leak is in the evaporator, you may need to repair it or replace it altogether. The repair entails cutting and crimping the tube to fix the pinhole ruptures. You may also want to solder the cracks. 

However, if you find that the puncture is quite extensive, you may need to replace the entire evaporator. A professional auto mechanic better undertakes this task. 

When the leak is along the lines, the following solution is ideal.

Use a super seal 

An AC super seal is an inexpensive and quick way to fix leaks in the AC lines. It can also repair metal AC components and eliminate any residual moisture inside the AC system. The R134a Super Seal is the most commonly used for this repair. This product contains special seals and conditioners that seal common rubber leaks. 

Steps to follow

steps to fix refrigerant lines leak
11 steps to fix refrigerant lines leak.
  • Locate the low-side service port under the hood and remove the cap. You will find it on the larger diameter aluminum tubing between the car’s firewall and the compressor. 
  • Turn the valve handle on the recharging hose counterclockwise. This action retracts the piercing pin. 
  • Attach the Super Seal can to the valve ending until it’s snug. 
  • Start the engine and turn the fan and AC knobs to maximum 
  • Connect the hose side coupler to the low side port. Ensure that the compressor is engaged and cycling on and off. If the compressor is not rotating, you’ll need to add refrigerant. If it does not engage and you’ve added refrigerant, you’ll need to take it to an auto repair technician.
  • With compressor rotating, turn the valve on the recharging hose clockwise until the pin pierces the can 
  • Rotate the valve in the opposite direction to dispense Super Seal into the AC system. 
  • Hold the can in an upside-down position until empty. 
  • Remove the hose from the low-side port and remove the can. 
  • Add more refrigerant to recharge the system.
  • Replace the service port cap and allow the engine to run for a continuous 30 minutes. This will allow Super Seal and refrigerant to circulate throughout the AC system. 

You will now start to enjoy cool air minus the hissing noise.

2. Leaky internal valve 

Hissing noise in AC can be caused by a leaky internal valve
The valves control the pressurization of the refrigerant.
Credit: mechanics.stackexchange.com

Hissing caused by a leaky internal valve will be slow at first, and then the hiss intensifies. It would help investigate a leaky valve as soon as possible because it will lead to more significant problems. The compressor valves control the pressurization of the refrigerant. If the refrigerant is not pressurized, air conditioning will not happen.

Mechanics recommend using the balloon method to detect leaks from the AC high-pressure and low-pressure valves. The balloon will fill up, and you’ll know there’s a leak from one or both valves.

Anyone can replace a leaky valve. However, if the leak is from the high-pressure side, you’ll need to evacuate the refrigerant charge because it is a ball-type valve, not a Schrader valve.  So, if you don’t have the proper equipment for removing refrigerant, then you’ll have to drive to an auto repair shop.

Steps to replace a leaky internal valve
5 steps to replace a leaky internal valve.

To remove the Schrader valve on the low-pressure side, you’ll need a valve stem remover. After taking out the Schrader valve, push in a new one. Then, test if all the air has come out of the system. After which, you recharge the system with the ideal amount of refrigerant (depends on how much refrigerant has leaked). Lastly, check if the AC system is giving off cool air. 

3. Too high pressure in AC compressor

Too high pressure inside the compressor can lead to hissing sound of AC
Another cause of hissing sound is because of too high pressure inside the compressor.
Credit: acprocold.com

Another cause of hissing sound is because of too high pressure inside the compressor.  High pressure is an indicator of several problems. One, the aluminum in the condenser distorts and causes leaks in the evaporator. If this is the issue you’re facing, you may have discovered it earlier, where we discussed how to diagnose refrigerant leaks. 

High pressure can also indicate a bad condenser fan motor. Debris could be blocking the flow, preventing or limiting airflow in the AC. You’ll need a technician to diagnose this problem and purge debris near the fan. 

You may also be dealing with an overcharged system. It means the AC system has excess refrigerant or oil. At this point, you’ll need to measure the system’s pressure. The readings on both the high and lower pressure side should not be above 150 psi when you’ve turned off the engine. 

If it reads zero or low, it means that the system is low on refrigerant. It’s still a good indicator that there are leaks. If the pressure is too high on both sides of the system – that’s above 30 Psi on the high-pressure side and above 250 psi on the low-pressure side, you will need to release pressure from the AC. 

However, releasing pressure is not something you can do at home. You’ll need a technician with the proper equipment since it’s an environmental and personal safety concern. 

Again, if the compressor is faulty and causing a high-pressure buildup, you’ll need to replace the entire compressor. In most cases, getting a new AC unit is better and cost-effective in the long run than replacing the compressor only.

A video about how to replace AC compressor.

Other AC sound problems 

1. Loud squealing or screeching noise 

The serpentine/drive belt is worn out or loose, which leads to a screeching noise.
A screeching noise is an indicator that the serpentine/drive belt is worn out or loose.

Another common AC problem is a screeching or squealing type of noise. This noise usually comes on when other systems are on. For example, the AC is on, the headlights, wipers, and a ton of other features are working. 

This noise is usually an indicator that the serpentine/drive belt is worn out or loose

The serpentine belt connects the engine to the compressor. It works to turn the compressor and lead the compressor to pressurize the refrigerant, thus cooling the car. 

When the serpentine belt is loose, it does not make proper contact with the compressor. Hence, the compressor will not turn entirely. It’s what causes the loud squealing noise. 

To diagnose this problem, you’ll need to check the drive belt physically. This action involves pulling and pushing on the belt to test its looseness. 

How to Fix: Steps to Take 

  • Locate the adjustment/tension screw on the serpentine belt. It’s a wing nut near the alternator. It’s hard to miss. 
  • With a socket wrench, place the socket end onto the wing-nut. Turn the wrench clockwise to apply tension. The socket needs to fit snugly so that it can tighten the nut. Note that you cannot tighten the serpentine belt by hand. 
  • Check that the serpentine belt is tensioned correctly by ensuring that it has only ¼-inch of play. 
  • Turn on the AC and listen for any whining noise. If the sound is present, you have overtightened the belt, and it can damage the alternator. 
  • In this case, turn the tension screw anti-clockwise to loosen it, then check that it’s at the proper tension. 
  • To ensure that that the drive belt is at the correct tension, turn on all your accessories. The squealing will be gone. 

2. Buzzing noise 

A buzzing noise is a symptom of an overcharged system. This issue often occurs after servicing the AC system. When there’s too much refrigerant, it results in a buzzing noise every time you turn on the AC. Overcharging can damage the AC’s components, and it can lead to a failing engine. 

The best person equipped to bleed the refrigerant is a mechanic. This is because the excess coolant is already causing a rise in pressure inside the compressor. Plus, refrigerant is a hazardous chemical. Some laws govern what should be done to remove excess coolant.

A mechanic will use airtight seals and vacuums to bleed the excess coolant. This system collects the refrigerant into an enclosed container for later use or recycling. This method keeps the refrigerant out of the environment. 

A video about how to fix the overcharged AC system.

3. Knocking or clicking noise 

A clicking noise is usually heard when you turn on the AC. It’s usually a low-volume sound. This sound is a result of the compressor clutch and the pulley as they engage. 

If the clicking noise is consistent and rapid, it can indicate a seized compressor, low refrigerant, or jammed pulley. This problem causes the AC to cycle ON and OFF repeatedly. It’s best to have a mechanic check this problem out. 

For a knocking noise, it shows a faulty component or a loose part. The loose parts like mounting bolts and other AC components shake when the AC is on. That’s why the knocking noise comes on. 

To diagnose this issue, all you need is to pop the hood, crank up the AC and perform a visual inspection of the various AC components. You will see which parts are shaking, then tighten them. You can also apply a thread-locking compound to prevent them from loosening soon. 

Read more: Why Does My Car Engine Knock On Startup Then Goes Away?

4. Rattling noise 

This noise shows that the engine is under a heavy load when the AC is running. At times the noise becomes louder when you accelerate. If this is happening, it shows a faulty compressor that is due for replacement. 

You may also need to replace the compressor clutch. This part will rattle when it becomes worn out. The same goes for the idler pulley. 

Replacing the compressor clutch, AC compressor, and idler pulley need a trusted mechanic.  

A video about how to replace the AC compressor clutch.

Read more: My Car AC Sometimes Works Sometimes It Doesn’t: Why and How to Fix

How much does it cost to fix hissing noise in car AC?

The first thing you’ll need to do is inspect your vehicle. If this needs to be done by a mechanic, it will set you back $100-$300. This price is inclusive of checking the belts, hoses, and connections and having the system recharged.

To fix minor leaks and recharge the system, most mechanics charge $150-$800. This process can include replacing parts such as sensors and hoses, including the compressor or condenser. 

Extensive repairs such as replacing all the AC’s parts like the belts, hoses, compressor, condenser and recharging the system usually costs $1000-$4000. This price depends on the car’s make and model. 


An AC hissing noise is quite common. As we’ve seen, it can be a normal function of the air conditioning system in a vehicle. However, if the hissing sound is accompanied by an AC that’s not getting cold enough, it’s a symptom of a host of problems. Some of these issues you can diagnose at home, while others require an auto technician. Either way, you know the steps to undertake when this happens. 

Please don’t wait until your car’s AC stops when you need it most. Diagnose an AC hissing noise problem before it’s too late

Read more: Engine Hot AC Off: What Does It Mean And How To Fix It?

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