I have a ringmod that does the same thing - roll off the guitar volume and I hear the internal oscillator (in this case it's a whistle). None of my guitars have treble bleeds. I tried everything with that pedal I could think of with grounds/PSU etc and nothing worked. One day I tried it with my Tele that has switchable active electronics and in active mode the problem disappeared. My guess is the pedal needs to see a certain impedance on the input. I ended up building a little FET circuit to sit inside the pedal and this fixed the problem.