I have one, its all rigged out. I have it set up with a spot-hogg hunter hog-it sight, limbdriver rest, paradox sling, octane stabilizer, G5 peed, d-loop, and also have string leeches on the string and it is still there. it seems like something that starts in the limbs, resonates to the riser and continues up, right to the hand in the middle and that is where naturally you feel it. epower100 is setting up the captain as we speak, and it has the same issue, so he is going to see what can be done to eliminate this.
the material in the string stopper seems rahter soft to me, as soft as that on duravanes string tamer.