[redlinedeath]

this may just be looked at as a stupid answer, but if you were to get a good microphone, and rig it somehow, so it picks up your exhaust tone, send it to sound editing software on a laptop that has lightning fast processing speed, invert thesoundwaves in real time, and play back the new inverted waves simotaneously witht he actual exhaust, it should theoretically eliminate the sound... completely. but as you cant have perfect real time recording/playback, it would still have a very very light sound. i know, stupid, but ive always found this concept intriguing. i may have the process wrong, but its something very similar... i wish i had large amounts of disposable income to try something like this... itd be fun!

and yeah, this process may not work in this application, but its still a pretty damn cool concept!