Like a kind of humming, grinding sound (not a sharp grinding though)? My new one does that too, I think it has to do with the Benq drive, my old 360 (Toshiba-Samsung drive) makes a grinding noise when I turn it on or use the DVD drive, I'm pretty sure it is a laser reset, where the worm gear drive is moving the laser head back into "not used" position or into "ready to read disc" position (whether that's all the way back or all the way forward I don't know). The new drives (my new console has either the Lite-On or Benq, which are very similar) sound much better than the old ones, but I'm thinking it is the laser head moving (can't really think what else it'd be, it makes the sound even without a disc so the only other moving thing is the head).
Just got my new one out and plugged it in (I've been using my old one since I did the mod), it doesn't really sound like grinding (much unlike the TS drive), it sounds more like a motor than a mechanism, may be that my drive is still new or it could be a difference with the Lite-On model. I've heard other 360's make weird noises (my friend's 360 has a Hitachi drive that makes a different noise) but they all make some noise due to the drive.
