Sounds a bit like you are reversing when you are in 1st/2nd at high RPM? I assume this is normal because the few other E85/E86s I have been in have had it, to varying degrees. As mentioned above, you are basically sitting on the whirry bits.
If it's a dry "clunk" when changing from 1st to 2nd or from reverse to 3rd, then you might benefit from getting SI B33 04 08 done: "The differential input flange mating surface with the input drive pinion is insufficiently greased."