When I play my MP3 player or Sirius through the radio's auxiliary input I get alternator whine. This is a Goldwing trait due to ground loops. There are a couple companies that make ground loop isolators for the GL1800. I already had a cable connected to the auxiliary input and wired into my Sirius. I ordered a Kennedy Technologies isolator from CycleGadgets since I could use it in conjunction with my existing cable. If you're starting from scratch Electrical Connection also has ground loop isolators.
The ground loop isolator.
The isolator installed. Very simple.
Update - Not only did the isolator get rid of the whine but I gained some volume. The Sirius has the volume at '17' that used to be '23'.