The "chhhhhh" sound:

That's the sound of your air dryer cycling and shooting out moisture. It happens when the air system hits its peak pressure, around 120 or so depending on the truck. If you have a bad air leak, that the pressure falls, the compressor kicks in, builds pressure to the max, then it fires. If the pressure is not dropping, you may have a bad sensor in the air dryer, or the air dryer itself may be bad. The canisters are replaceable, and if it is saturated may be the problem too.

 

I experianced the same sound and later discovered a hole in the main air line 7 inches from the air compressor. I fiqured a hole the size of a needle became the size of a quarter within about 8000 miles when I finally was able to see what it was I had been hearing for three weeks. Not saying that IS your problem, but worth a look.

 

If you air pressure is staying high all the time and not cycling the problem is likley the compressor governor , the round 1 1/2 or so cylinder bolted to your compressor, they are cheap and easy to replace , any hd parts store ,truck stop dealer should have one they are pretty much common

 

Your air governor might need a new regulator.

 

It could be the dryer but it sounds to me like for some reason the air governor isn't working anymore. The sound you are hearing could be the safety valve opening to bleed off excessive air pressure. I would get it checked out right away if I were you. Maybe you can idle the truck up and listen to see where its coming from. If you decide to do that, wear eye protection and do not use your bare hands to check for leaks.

 

really bad air leak somewhere. turn truck off and you should hear it.

in my case it was the fan clutch. leaking air like a mother.

 

Isn't checking the compressor part of the pre trip? (Yup, it's still there officer.)

Tracing the airlines shall reveal it's location.

 

need to have the air system checked out from the compressor to the dryer, could be something as simple as a pin hole in your signal line from your gov to your dryer.