We have a smart community!
Now, Joystiq compared the sound of the Xbox360 to an old fridge. Yes, you do hear sound and many people think it is due to the fans inside...but, discussing this with others, we've come to the conclusion that is it NOT the fans, but it is a DVD spinning in the drive.
Well, a smart community user brought this to the attention of the community.
I believe this theory. I do not have any data to confirm/deny this, as this is my thought, and not based on any information given privately.