Yup, the audio file too sounds like a ground-loop (from USB specifically, it sounds like to me)
Have you made any other connections on the Rev2? Like USB? If so, try disconnecting the USB cable to see what the result is in the audio. If the problem disappears, use MIDI DIN instead of USB, that usually does it. If not, try a stereo DI box like Josh proposed. It's kind of "hit-and-miss", in some cases it works.