Help.. I'm getting strange noises every time I move my expression pedal??

I use my M9 primarily for Delay/Reverbs and every effect has the mix parameter controlled by the expression pedal. For example... in the heel position on the Cave Reverb every parameter is set except the mix is at zero and in the toe position all the parameters stay the same except that the mix level is dialed in to 50%.

So when I get to a part in a song that I want reverb I push the expression pedal to the toe position and it makes this warbly screeching noise for a second instead of just fading into the reverb setting. It is very noticeable but only happens when I've had the mix at zero for a while and then push down on the exp pedal. When I first turn the effect on it will not do this but after I play a few notes and adjust the expression pedal from off to on I get this sound as if I'm adjusting the feeback knob on a delay pedal.

Any suggestions is greatly appreciated.. thanks!