My 250 gig 360S does this once or twice a week. I shut it down and the disc drive spins up extremely fast and the power button blinks slowly. Usually, I ignore it and it stops after a few seconds, but sometimes it shows no signs of stopping, so I power back
on and shutdown again.
This can't be good for the system, the disc drive sounds like its spinning about 10x faster than it does when playing a game off the disc.
I haven't been able to find a good answer as to why this happens.
I even tweeted the Xbox support people (usually very prompt and helpful, by the way) and they suggested it might be the fan.
It's clearly the disc drive, because when I hit the eject button the noise stops before the tray comes out.