2008 Chevy Silverado 2500HD 6.6 Duramax with Allison Transmission 6 inch lift. I have had a vibration that has been looked at by 2 different mechanics now. The noise is new and hard to explain. Noticed it when the truck is at operating temperature and the transmission temperature was 144. The noise sounds like a bunch of stones hitting the underneath of the truck. (No I was driving on pavement) Most notice it between 30 and 40 mph. Mechanics have checked the real axle, driveshafts, universals in both shafts and have checked the transfer case. It has both mechanics and myself stumped. Any and all thoughts would be greatly appreciated. In the photo below, it's the truck on the right. Also, in the winter I run 285/75/18 tires and the summer 35x12.50x20's.