looking for some pro help. A freind of mine has duramax diesel that has been making a jake brake noise for the last year,when the truck's under load, but not very loud.(this is the friend) if i have the stereo on i dont even notice. but its getting worse, i can feel vibration in the throttle pedal first, now i can feel it in my seat, and its louder. chevy dealer has no clue they think its my a/c, but it does it with it on or not. i dont want to get stuck on the side of the road. got any ideas of what it can be.by the way it sounds more like an air commpressor loading up. thanks for any info