I have the same problem!
it started last weekend… any time the qu-drive light indicator flashes (while recording to usb, when first plugging in a drive) we can hear static through the st3 input, when recording to multitrack its a steady pulse about 80bpm
when recording stereo its a very fast buzzing
never happened before
muting st3 will not suffice as we often will be playing music through that input while recording to usb
any ideas?