I have the same noise on my 1gen MS3! I just noticed it last month and haven't been able to figure out what it is. It almost sounds like something is loose and just bouncing around in the engine lol. I think I saw a TSB about this issue, but unfortunately it was only to about 70k miles. I'm at 95k so I don't want to pay for a dealer to just look at it if I can figure out the issue myself.
it may be a tsb, but i'm gonna try to push on some parts this afternoon or tomorrow to see if i can narrow it down. if it's the serpentine belt idler pulley, or tensioner, ive read it's not covered under warranty. i have 50k miles. =(