My JD Crybaby Mini wah screeches in the toe down position if I'm playing with a distorted sound. I didn't manage to turn the pedal off at my last gig (though I though I had) and kept wondering where all the feedback was coming from. As Fretworn rightly says, it's just the high level of boost at high frequencies plus a possible overload of the Line 6's A/D converter resulting in some digital clipping as well. It may also be stimulating any microphonic tendencies your pickups have.