shudder thump

Well I did it, I bought a sable. Its an LS Premium with 24V DOHC motor. Test drive was great. I get it home and goto back it out of my driveway and while in reverse before shifting into drive, foot on the brake, car stopped, I hear this thump thump thump kinda in tune with the motor?? I put the car into drive, all is normal. Shift back into reverse, normal as well. I'm confused, I'm sorta thinking motor mount may be bad somewhere , but I couldn't see the mounts from the top of the motor? Has anyone else run into this? Also, when it shifts into OD, it kinda almost feels like the motor thunks, same thing if you goose it hard from a start and it shifts into 2nd. Car has 36K miles on it, tranny fluid is full, doesn't smell burnt either. Need some help, hoping I don'thave a tranny on the way out......(thats why i'm selling my stratus with twice as many miles on it!!)