Hey guys i got an 05 GMC sierra 1500 z71 with a weird rattle coming from around the transmission. It's very loud when driving, sounds like something's so loose its gonna fall out from under the truck! It really sounds like axhaust or a heat shield, but I've checked everything from the header bolts to the tips and the exhaust is rock solid. What leads me to believe its drivetrain related is that it only does it in the 1500-2000 rpm range, when driving or reving in drive with my foot on the brake. It will not do it in reverse, whether moving or foot on the brake, or in park or neutral. When I realized it wasn't exhaust I immediately thought torque converter bolts or cracked flywheel, but it wasn't that either. You guys have any ideas on what I should check next? btw I have a lift in the shop at my farm, so I can inspect up under there pretty good, been looking at it and checking bolts all day, can't find anything wrong.