It's signal interference. Happens all the time on my set, but only when the cell is in use.
Probably picked up through the speaker wires not the speakers themselves. The more wires you have, the more potential sources of interference.
I don't think there's too much to do about it, unless you want to shield all those wires.
I'm sure someone knows a better way.
Edit: And it's not feedback. Feedback is signal that loops from source to output and back to the same source in a way that the signal grows in strength with each loop.