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air conditioner hissing noise

Why Is My Air Conditioner Making A Hissing Noise? [5 Causes & Solutions]

Nowadays, air conditioners are made of high-quality materials. They are not prone to breakdowns. However, it can go wrong at some point still. When it starts making a hissing noise, it could signal that something is wrong with your AC. 

In most cases, the hissing noise from an air conditioner is caused by a refrigerant leak. High compressor pressure and leaking ducts could also be to blame. Normal thermostatic expansion valve operation can also produce such a sound.

This post summarizes the five most common causes of an AC hissing sound. Reading it will teach you how to deal with it in a matter of minutes.

It is important to note that you should address this sound as soon as possible because it could lead to much more expensive repairs in the future.

1. Refrigerant leak

refrigerant leak

Nine times out of ten, the culprit of an AC making a hissing noise is the refrigerant leak. 

A refrigerant is a magical substance that absorbs heat in the air by changing from a low-pressure, low-temperature liquid to a high-pressure, high-temperature vapor. The refrigerant is easier to boil when the pressure is low.

As a result, the tubes through which the refrigerant travels must be able to withstand high pressure over time. However, the tube can crack due to wear and tear, corrosion, extreme weather, or other factors, and refrigerant will leak out through the small holes.

Because the refrigerant is pressurized, it will make a hissing or bubbling sound as it exits the hole. This sound can even occur when your air conditioner is turned off. In addition to the hissing sound, you may notice an increase in your energy bill or the evaporator coil freezing.


You are not allowed to recharge your air conditioner on your own. Because the refrigerant can endanger both the environment and our health, this task should be performed by a certified HVAC expert.

The professional can detect the leak and determine how much refrigerant your air conditioner requires. Usually, it will only take about half an hour to get it recharged.

However, if your model uses R-22 Freon as the refrigerant, you may consider purchasing a new unit because R-22 is outdated and costly.

2. Normal thermostatic expansion valve operation

expansion valve operation

The thermostatic expansion valve has the ability to regulate the pressure and temperature of the refrigerant liquid before it enters the evaporator coil. The pressure of the refrigerant is reduced after passing through this valve.

When it tries to relieve the pressure, it will make a hissing sound. That is, this is simply a sound produced by normal operation.


Fortunately, nothing is wrong with your machine.

3. High pressure in the compressor

The compressor raises the temperature and pressure of the refrigerant vapor. However, if the pressure inside becomes too high, a hissing sound may begin to emerge.

This could be due to the high temperature of the refrigerant, which can occur when the weather is excessively hot or humid. It could also be the result of more serious issues with the compressor.

If you trace the source and discover it is coming from the outdoor unit, the compressor may be broken.


Fixing or repairing a compressor could be prohibitively expensive because the compressor is the core part of your AC. 

If your machines are relatively old, replacing them with new units may be a better option. If your machine is still under warranty, contact the manufacturer.

Aside from that, new models will include a pressure sensor inside the compressor, allowing it to shut down before the pressure becomes too high.

4. Ducts are leaking 

Given that the blower continues to draw the air in, it will create high pressure in the ductwork. Like the tubes in the AC, these ducts can crack or break for many reasons. 

When there is a hole in the duct, the air will leak, and because it is pressurized, it will create a hissing or high-pitched whistling noise.


It is difficult to pinpoint the source of the leak. The duct system may occasionally have a few leaking points, some of which may be hidden in the wall.

You can fix it yourself if you can locate the source of the loud noise and it is easily accessible. Otherwise, hire a local contractor to locate and repair the leaks.

5. Malfunctioning heat pump reversing valve 

To cool the room, some families use a heat pump rather than a traditional air conditioner. The main distinction between a heat pump and an air conditioner is that the heat pump can both heat and cool the room.

There is a reversing valve that determines the flow direction of the refrigerant, which determines whether the heat or cool mode is activated.

When this component fails or becomes stuck, the machine makes a hissing sound.


The main cause of a reversing valve failure is a faulty solenoid. Fortunately, it is a simple fix. However, if you lack the necessary expertise, you should avoid doing it yourself.

How to prevent the hissing noise from AC from coming back?

Now that you understand why your AC is making a hissing noise, you may be wondering how to avoid it in the future.

While those leaks are more likely to occur as your machine and HVAC system age, you can have your AC serviced on a regular basis to reduce the cost of repairs.

The experts can detect the problem early on, limiting the damage to other parts of your system.


You may feel anxious when hearing a hissing noise from your air conditioner. The main culprit of this sound could be a refrigerant leak. You need an HVAC expert to locate the leak and recharge your device. Don’t try to do it yourself because the refrigerant is harmful.

It could also be caused by duct leaks and the defective compressor or reversing valve, which needs the assistance of a pro as well. 

There is one exception. The thermostatic expansion valve can make a hissing sound when working. In this case, you don’t need to do anything. 

I hope this guide can help you out!

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Liz Yang is the founder of Airsmartly. She has been working at home for a few years and realizes that the performance of the HVAC system plays such an important role in our life. She has tested more than 150 products in person, including humidifiers, air purifiers, dehumidifiers, and ACs, and wants to share tips about using or troubleshooting these products with you. Her uncle is an HVAC expert with over 30 years of experience in the field, and often offers assistance when she is unsure how to handle a situation. He is also in charge of reviewing the articles on this site.

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