When I take my 2000 bonneville out, when I floor it either from a stop or from 30mph and when its in first gear at max rpm before it shifts under full throttle as soon as it shifts into 2nd theres a thud thud sound. Its a weird noise, I can actually feel it, its like hitting 2 really small speed bumps. Almost feels like running over 1" tree branches. The sound is always in a set of 2. I'm thinking maybe a trans mount? Which one would cause this?
thanks
craig

Check them all. Front drivers side, rear drivers side, and passenger side of the engine. There is also a mount off the diff cover.