YESSS!!! SOlVED IT!!! this has plagued me for a year. literally. Ok, what happened was this:
all of my other audio plugs have rubber shielding directly outside the metal plug. This one plug does have the shielding, but it starts up a little ways. basically, plugging the wire in all the way was causing it to pick up signals from the case. If i pull it out just slightly (not enough to affect the sound quality any) it stops. I can't believe it was that simple!