HD clunking away for no reason...

OK, my video computer is the only computer I've noticed that'll do this. If I let it sit for about 30 minutes or so without doing anything, the HD starts grinding away. Nothing is happening on the screen and I don't have any virus scan or anything like that. Actually, all it has is Windows XP, Photoshop, Premiere, soundfourge, and deepfreeze. Well, as soon as I tap the mouse, it stops thinking and it's good to go. Obviously, this isn't causing much of a problem, but it's annoying when I'm trying to sleep and it starts making all this noise I know I can shut it down, but sometimes I like the hum of the comp to help me sleep...

For those who don't know, DeepFreeze restores your computer to however it was when it was installed/frozen. So basically, the computer has only been rebooted once since I loaded windows. Could this cause the problem?