Tip of the day - Noisy XBox 360 Lay it down
Not all XBox 360's are created equal and it appears that some have noisier fans than others when in Media Center extender mode. One thing I have definitely noticed with our 360 is that it is a lot quieter when laid on its side (horizontally) than when in the vertical position. Not sure why this should be but, from reading a load of web posts, it does seem to be a common feature.
So if you have a noisy 360 lay it down.
IMPORTANT - never move your 360 with the power on and disc in the drive. if you do you may very well scratch the disc and even possibly damage the drive heads.