btw...i ended up getting an asus p5k-se with a Core 2 Duo 6300. C2d is awesome but i've seen a friend's pc same specs as mine except a different mobo every component running at stock speeds operate much smoother and quieter...i have no idea whether the case doesn't offer sufficient dampening but its a thermaltake soprano vx and everytime i start a new task it makes a hell of a racket like my old PCs back in the day when pentium 2s were all the rave....
What sort of noise is it? Can you record it? Silencing PCs is a somewhat hobby of mine, if it bugs you I can help you solve it.
to me it sounds alot like the chassis has got a loose screw which i dnt think is possible.could also be a fan.but then agen im not a cooling expert.i've just thought it may be that the chassis is not placed on a firm enough foundation.will explore that option too...
Is it a harsh roaring sound that disappears a minute or two after the PC's been on? If so, it's probably a damaged or faulty bearing in one of your fans.
hmmm....that seems sorta the problem descripiton except on and off it keeps coming back. but particularly at startup..whats the solution?
try turning on your PC with a certain fan unplugged and see if the noise stops. That goes for all fans, including the power supply one (which you can't unplug, just shove something non-metal in the blades when it's off to stop it spinning round, it won't do any harm to do it for just a few seconds)
if usually does it at startup after being off for several hours then the fan needs lubrication. usually is the psu causing the noise i've found but have seen both the cpu, videocard & case fans do it too.
PSU and Graphics card fans seem to do it the most often since they're usually exposed to the highest levels of heat, but it can happen to any fan, so check them all.