I'm not sure yet, I was driving about 35 nice and easy, I felt what I thought was I drove over something hard and looked in the rear view mirror for something on the ground and turned around to find it. I didn't find anything but as I gave it some throttle very easy there was a very pronounced thunk thunk thunk. I would still move, so with just coasting and super light on the throttle to try and keep it moving it would thunk thunk thunk. Let up on throttle and coasts smooth, just touch throttle very slightly and thunk thunk thunk. At first I thought I had broken the snout off the Torque converter, pulled the transmission and torque converter looks fine. It was a slow thunk thunk thunk so I pulled the differential and it looks good. The driveshaft looks good and the u joints look good. Any suggestions.? Oh, and all the mounts look good. Some slight mods on the engine, just guessing 350-375 HP.