Hissing Noise Under Dash When Ac Is On? Common Causes You Should Know

Ever noticed that concerning hissing noise under dash when AC is on?

While it is true that no air conditioner is ever totally quiet, there are some noises you may hear that can alarm you.

After all, a constant – and unexpected – AC noise is usually a symptom of a much bigger problem. So, if you are noticing such sounds, it is important to get your AC checked and fixed by an expert right away.

Find out just what it means when a hissing noise and a few other noises are heard when you have your AC unit on.

hissing noise under dash when ac is on

Reasons Behind A Hissing Noise Under Dash When AC Is On

If you hear a hissing noise under the dash for only a few minutes, and then it stops after a few seconds, then you should not be concerned at all.

This could only mean that you are hearing your refrigerant gas located in the expansion valve.

However, if the sound is more persistent, it means that you have a leak in the refrigerant. This leak could come from the internal valve or the refrigerant lines.

It is also worth noting that leaking refrigerant lines produce some kind of a bubbling noise. When you leave this issue unchecked, this will cause the hissing noise to intensify and cause further problems.

This is why you need to make it a priority to get your leaky internal valves checked and fixed sooner than later. This way, your AC will continue to produce cool air while preventing the leak from getting worse and being beyond fixing.

If you think there is a leak somewhere, the most immediate thing to do is to turn the AC off. Otherwise, you may end up inhaling freon, which is the refrigerant gas in AC units. This is dangerous to inhale, which is why you should shut the unit off when you hear a hissing sound.

Another possible reason behind the hissing noise is the pressure in your air compressor being too high. It usually starts off as a hiss, and then turns into a shrieking, high-pitched sound. In the same way, you should turn your AC off when you hear any hissing or whistling sound.

Then, be sure to leave the AC unit off until you have the right specialist look at the issue.

Other Possible Reasons For A Hissing Noise Under Dash

In addition to the reasons above, there are other potential causes of such hissing noise.

When there is no cool air coming out of the AC, it could mean that there is a deeper problem. The compressor may be cycling as it should and you have the aluminum metal tubing located in the firewall getting consistently cold. However, if there is only ambient air temperature out of the vents, then it could mean a few things.

One, it may be an evaporator issue. But there are a few other reasons why this can happen, which is why it is best to have a mechanic test for a leak in your AC system.

It is also worth noting that newer vehicles have this type of problem where there is the AC and clutch relay located. Thus, the AC tends to over-pressurize, which causes the relief valve to open and release the refrigerant. The battery can also drain when you have the engine off.

If your vehicle is still under warranty, then you should be able to have the AC repair cost reduced to a lower price. But you need to make it a priority to take your vehicle to an expert mechanic since this can be a sign of a more serious problem.

Now, if the compressor still cycles on and off, it may be due to a stuck clutch that can be preventing it from cycling off. When the evaporator is leaking or faulty, this would cause the refrigerant to leak out and the compressor will turn off completely. The pipe should also not get cold, which means there is no cool air coming out of the vents.

You may want to place a set of gauges on your unit to determine that charge. From there, you should be able to figure out the next step to take.

After a visual inspection, and you do not see any leak, then it may be coming from a blind spot or behind your firewall. This is why it is always good to conduct a leak and pressure test to make sure that everything works just as it should.

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Final Word

When you hear a hissing noise under the dashboard when your AC is on, so many things can be causing this. Your evaporator may be faulty, there may be a refrigerant leak, and your compressor may be off and not working as it should.

No matter what the cause may be, it helps to have the proper diagnosis done to get this problem sorted out and prevent a more serious issue from occuring.

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