some wah's have a cutoff, if they detect inputs of lower than a certain threshold, it'll cutoff all output. however, before that occurs, u'll still hear the noise coming out from the pedal.
the pedal you're using is particularly noisy =P you COULD try getting a noise gate after the pedal to reduce the disturbance, but other than that... turn off the hi-gain on your amp? (you might not like that...) mm... mod it? heh...
well don't worry, as long as you keep playing, the notes should cover up that noise very well, and sometimes there IS NO noise when you give an input of significant level, cuz the noise comes from within the pedal, trying to change signals that aren't clear (static, etC)