It's been a while since you posted this, but which XM radio do you have? You're probably just hearing the transformer vibrating because it's not securely mounted inside the power supply - maybe it was dropped or fell out of the wall plug at some point. I can't see how this could cause any problem, but if it's an annoyance I'd call Delphi, Audiovox or whatever company made the radio and ask for a new power supply.