Hi I have 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.