This is an odd situation, that I think is related to the torque converter. The second I release the key on start up, I get a thud sensation in the drivetrain. It actually causes the truck to lurch forward slightly, while In park. feels like someone rear-ended me at a slow speed. This happens about every 8th start up or so. I took the truck in to a local gmc dealership, and got the typical "we can't replicate the problem." Well, this continues to happen to this day. I'm concerned this will cause a failure in the transmission/torque converter. I have had no problems with the transmission shifting. Its actually been shifting flawlessly and smoothly. I suspect I will eventually encounter rough shifting, or a failure of the transmission/torque converter, related to this situation on engine start. I now have 26000 miles on the truck. I love everything about this truck, except the unusual problem I'm having on start up.