> Power the machine up with the case open, when the noise starts, carefully reach in
> and touch one of the fans (near the center, although CPU fans don't turn nearly fast
> enough to do any damage). If the noise changes or stops, you'll have your culprit. My
> money would be on the graphics card fan.
This is how I find noisy fans. Note that the danger isn't that the fan will hurt your finger, it's that your finger will hurt the fan given how thin and brittle the blades on many of them are. So be careful