Well, time to pull the cover off, and run it like that for a while. When it starts buzzing, do what Hex said, start putting pressure on the fans, see if it stops, or see if you can make the sound happen by playing with the fans a bit. Be careful though, like he said, those suckers can take the tip of your finger off pretty quick, and human blood will conduct electricity, (on a side note). When you've isolated which fan it is, it may be a simple matter of tightening a couple screws, if it's a case fan or chipset fan, or remounting your heatsink and fan, if it's the cpu fan. Let us know.