Hi, for our home theater system we have a jbl sub that started this obnoxiously load hum in the middle of the night while I was sleeping. It sounds like the hum of a transformer but way louder. I've opened the unit up and had a look inside where i found nothing (or at least can't tell) wrong. It is a powered sub and as soon as you plug it in it immediately starts buzzing very loud whether the volume is high of lowest and whether there is any audio sources connected or not. So if anybody has any ideas about why this might be happening, I'm all ears.