Occasionally when using my 20" iMac, I hear a high pitched squeal from my speakers. Usually turning the volume up one or down one "plop" using the keyboard stops it for a while. Does anyone know why something like this happens? Could it be my speakers, or the iMac itself? I tested the same speakers on my PowerBook, and it doesn't produce the squeal. I take the bus, so I do not want to have to lug this iMac on the bus to go to the Apple store (Even though I took it home on the bus ) This iMac is almost brand new. Also, it doesn't happen when there are no speakers plugged into the iMac, so the iMac speakers are fine, it only happens when I plug in my external speakers, and the squeal comes from my external speakers.