it's not making any sound when you do *anything*? I mean, watching DVDs, image/video editing, nothing?
I find that my fan doesn't usually come on during my normal web browsing, chatting, music, writing with Word, but it'll come on after a while when I am watching a DVD.
Aside from that, the Fans (on my powerbook) usually kick in when the processor is around 135-140 degrees farenheit. Do you know how hot your machine is running?