my PowerBook plugged into the ac plug, it is sitting on my desk, no other speakers are turned on around it. I've had this set up this way for weeks now. I opened Garage Band today and there was a huge screach so bad and OSX locked. I had to power down. I opened GB again and same thing. This time I was able to turn monitor off and no feedback issue. It was so loud someone thought it was a smoke alarm gone seriously wrong. I do not know what is happening. Nothing else is plugged into the PB.