my advice would be to add as many mufflers into the system as possible. I feel I was able to remove the rasp finally by adding an extra muffler into the magnaflow cat-back system. I posted some detail on it in a thread, do a quick search and see if you can locate it.
2003 BMW 330i ZHP
ESS Twin Screw Stage 2 - Wavetrac - Headers - Magnaflow - BMW Performance Intake and SSK
Turner Motorsports Reinforcements - H&R Spacers - Powerflex - StopTech