Strange crunch when pedal is off, Any Ideas?

i recently got an arion bass distortion pedal, the thing sounds great, but eats 9 volt batteries like crazy, so i went out and bought the 9 volt dc adapter that it needs to plug into the wall, this is where the problem is, with one of my basses, the plugged in pedal works just fine, both when it is on and off, with the other bass, there is a strange crunchy sound when the pedal is off, both basses have passive p-pickups set up in them, neither has any issues with electronics, any ideas? thanks