edit: Solved. Thanks guys
In: Engineering
I responded, but for some reason it was removed. My answer was simplistic,a nd answered the question.
The noise is from the air dryer expelling moisture from the air system. Trucks use compressed air for brakes and other functions. That requires that the system have as little moisture as possible. They have an air dryer plumbed in, and as it removes moisture, it has to empty itself sometimes.
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