It's usually one of two things: 1/ An electromagnetic/mechanical effect in the transformer in the inverter. You can usually stop it by carefully undoing the ta

It's usually one of two things: 1/ An electromagnetic/mechanical effect in the transformer in the inverter. You can usually stop it by carefully undoing the tape around the transformer and giving it a squeeze. 2/ The audio system is routed next to a bus somewhere and it is just plain old noise being pumped out of the speakers.