you sure this car is not an old audi? just kidding.

i would be very surprised if it is body flex. the unreplaced motor mount might be failing in tension. if you jack up the motor does it lift off the mount that you didnt replace?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.