Pioneer told me it was from "faulty grounds on external amplifiers." //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif
So, quite a few VERY EXPERIENCED PEOPLE have had the problem, I would have thought one of them would know how to run a ground wire.
My guess as to what caused it is simply a design flaw of some sort.