Rattling noise when I first turn computer on?

I've started to notice a kind of rattling noise when I first turn my computer on and after about 20-30 seconds it goes or away is very quiet. I opened up my computer and I can't really tell what it's coming from. Any ideas?

Hard disk, fans, cdrom. Sounds like something isn't screwed down tight. I would say the most likely thing is a fan. Might give your fans a nudge to see.jcknapier711

My guess is its a cable thats hitting a fan and as the fan speeds up the cable is pushed away, if it was anything else I'd suspect it would continue to make sounds.

I would be checking the fans. Mine does the same and I am just to lazy to change the fan till it dies.