I have an '04 Silverado 2500HD CC 4x4 6.0L that I have only had about a month. At takeoff I sometimes get a vibration, at about 1800 rpm usually in 1st and 2nd only, that sounds like a rear heat shield. I was under the truck last night tapping and banging to try and find something that would be causing it and couldn't find anything. Has anyone had a similar experience and found the cause?