Re: Rattling noise
I also think it's a fan. My old PC did that and I always thought it was normal routine so I just left it but coincidentally the PSU died on it last week so I guess that might have had something to do with it.... could be your PSU fan or something but I'm not sure. Also, cleaning out the dust inside the case wouldn't be a bad idea. It never hurts to go in there and clean up a bit.
Desktop: Dell Dimension E510, 2.5GB RAM, Intel P4 3.0GHZ, 250GB HDD, Windows Vista Premium. Might not be the best, but the old girl is still running good.