I've traced it to the stereo buss in jacks. It happens whenever anything is connected to them. I don't know if I've had it before (I've never really used them). Any idea how to fix?
Have you already tried using an isolator in case it's a ground-loop issue? The Behringer HD400 is good as it has 1/4" jacks, but even the cheap 3.5mm ones off ebay will work given the right adaptors.