Could also be the clutch and flywheel chatter. I know I get it and I have polyurethane motor mount bushings. My clutch has 85,000 miles on it with about 240 autocrosses too.
__________________
Sponsored by: KAM Racing Sports, Falken Tires, Progress Technology, Brady's High Performance, Taggart Performance Engineering, Rotora Brakes
Autocross is: 90% driver, 5% car, & 5%
CRAZY MOJO!
Autocross Help Page