Very occasionally the screen on my iMac (early 08 24") goes blank and it makes an incredibly loud static sound that’s independent of the volume control setting. The power then needs disconnecting to restart the machine. Haven’t been able to tie it to any specific program. Apple Care asked I leave it on logged in on a new user account (at the time I originally thought it only happened in sleep mode, but have since discovered this is not the case). It’s happened maybe 3 or 4 times in the same number of months. I’ve only found one similar case on the net, with someone suggesting it was Safari 4 beta, which I’ve found not to be the case with mine. It’s on extended warranty. I think there’s something seriously wrong with it (it’s already had the hard drive and power supply replaced).