Macbook makes funny noises

I've not got any messengers running. I know what a HDD click sounds like and it's not that; the noise isn't mechanical. Hasn't happened for a while now, but i know it'll be back.

I've just now muted all sound to see what happens.

Any other ideas? Generally I'll be running Firefox, Entourage, Word, Parallels and the Dashboard in the background.
 
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