The main xbox fan is speed controlled.
It could still be the fan making that noise. Xbmc has a setting for the fan speed.
You might turn it up and see if you get the noise as well.
If your xbox is a 1.0 version, it will have a little fan on the gpu as well.
Sometimes, a much fragmented harddisk or a harddisk on it's way out can make noise as well, altough it's more
a clicking sound of the heads being brought into position. Some harddisks even have a media streaming setting in their firmware. If you turn it on, the heads will move slower (making the harddisk seek time bigger as well.)
Due to that, they make less noise, and to stream media, it's not really a problem.
Sometimes, a coil in a switch mode power supply can make noise as well. Usually fixing it with a drip of silicone or hot glue solves that issue.
regards.