My automatic transmission is making a strange nise. There is an intermitant thumping noise comming from the transmission area while in drive. The noise is more more prevalent during de-acceleration. Also, when stopped, and the parking brake is on, and the car is in reverse gear, and I reve the engine just enough that it doesen't overcome the parking brake, there is a continious dull thumping or grinding sound. It only does this in reverse. This all seemed to start when I went over a train track and normal speed. Right after the car came down on the springs I heard a loud smack type noise. I've looked under the car but can't really see any thing. Could this be the U-joints?