One of the most complained about problems at my local dealer is a "noise coming from the back of the truck". I kid you not. These folks either forgot they had or did not know about the air suspension.
As I understand, the air will kick on anytime the truck feels it is not level and release when it is. Thus you hear it often.
The quick fix, and I need to mention this on another thread, is to get a louder exhaust
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Black Lingenfelter Lux