I have a guitar amp that if you turn the guitar volume down and the amp volume up, you can hear bbc type radio or Japanese? radio. I believe it's caused by magnetic interference. There's a song by the Police called Does Everyone Stare off the album Reggatta de Blanc that has an opera singer unintentionally comes in from the interference. They decided to leave it in the mix. The lack of a line signal allows the reception to be audible.