Been to the dealer twice for this, nothing found. At first they thought it was a heat shield vibrating against the fuel tank. When it shifts into 4th gear and above, at about 1,000 rpm, with no positive or negative load, I am getting a "corn sheller" sound in the TC or tranny. Won't do it until 4th gear then same in 5 & 6. Truck is a 17 2500 4x4 with 30k miles started doing this about 25k or so. Annoying as hell, and it sounds like something that will get worse before it gets better. Makes the same sound in 4 wd or 2wd, so probably not the transfer case, Any ideas ?