I have one thought that came to mind from experience: I had a magnaflow muffler and it would do that crap at certain RPM's. It was a internal baffle that had came loose and would rattle. I am not to sure how the stillen exhaust is but I thought I would just throw that idea out there.
I know exactly how you feel from rasp. It drives me CRAZY! That is why I was so strict on the exhaust system I picked out. Good luck bro
