A longer exhaust system helps with quieting the noise, as does simply having the system end
behind you;
The final extension on my SBP expansion chamber exhaust is just a length of heater hose, takes a lil bit of the "bang" out of the sound - but I put it there so it wouldn't spew on my packs/kickstand lol
A bit more work, but this helps quite a bit;
http://motorbicycling.com/showthread.php?t=10967
Also - the gear howl is quite loud on these things, Joe mentioned gluing a mouse pad to the cover - that is prolly the most effective, but I just took the cover off and sprayed the inside of the cover with "rubberized undercoating" (after cleaning it ofc). While that does help - keeping those gears
lightly greased quiets them the most lol
But as Joe said - the tensioner is responsible for a surprising amount of noise vibration - aligning it to the chain is critical for many reasons but noise is a biggie
http://motorbicycling.com/showthread.php?t=11815