I own a 2007 Sierra Classic, 2500HD, VIN 1GTHK23D57F166903. It has an audible vibration in the power train, most noticeable at the 55-65 MPH range. I'm told this is a normal characteristic of the truck. Quite frankly, I do not believe that. What is you corporate mailing address so I can send the specifics to you, in writing? Thank you.