When I turn my PC on, or resume from standby if it's been off for a while, my PC makes a noise like a wire caught in a fan, or a similar noise. If it's been going, I put it in standby, then turn it back on, it doesn't make the noise. Once it's been going 30 seconds the noise goes away, or at least becomes very very quiet. It's a self built PC. I can't locate the source of the sound by sticking my head in there, the sound echos around a lot.

I've checked them all though, and can't see anything, and I can't tell where it's coming from. I just put a fan controller in last night which controls my two CPU fans and front and rear case fans, and changing the fan speeds doesn't change the noise. The only other fans are power supply and  graphics card.

The only moving parts are fans, so i'm pretty confused. Anyone have any ideas?