[JohnM]

Looks like a feedback path in the soundcard mixer, for example the input signal is not muted in the output mixer. REW won't be doing that for you because you haven't selected an input and an output, so you'll need to do it manually. The only signal which should not be muted in the playback part of the mixer is WAVE, mute everything else (if the MobilePre mixer is not too clear on that try looking at the Windows mixer settings to see what they show).