One way to get to the bottom of noise related issues is to take a video documenting when and how it makes the noise and then post the video on youtube and give us a link to see it.
I put a straight pipe on my M9 (and left the muffler on the 105). I can really hear the turbo spool up in the 9, usually about 1700 rpm with no load and 1000 rpm under load. Kind of hums as well. Nice sound really.