Odd, im sure my iPhone is 1 at a time.
Anyways, interference. The only time I hear it is when im charging my phone and trying to use the aux in, I hear a whine that increases with RPM, its just feedback same as you can get in a badly setup sub system, not sure why I get it, everything is earthed correctly but its only slight and I just have to not charge and play music through the jack at the same time.
There is a little bit of interference sometimes caused by the tape drive motors, but there's sod all you can do about that really.