Thanks for the reply. This is the conclusion I have come to as well – however if I push the expression to to the toe (+2 octave) and litterally just tap the strings with my finger, so gently that no real audio sounds you still get the rustle/fizzle of the string being tapped.
To ensure it was not a grounding issue etc on the bass I have tried another instrument of a similar style plus an instrument not grounded to the bridge (DeArmond Ashbory) and all do exactly the same thing.
I have found that reducing the bass frequencies before the pedal reduce the level of 'distortion' (so taking the octave down effect away reduces it, rolling back the bass on the instrument reduces it and lowing the gain trimpot of the channel on the line selector it sits in reduces it also – But at volume it is always still there. It's off that the bass frequency is related as my cab uses low pass-ed speakers (LPF at 1khz) and has a mid-driver and tweeter for everything above that (in the same way you'd use a PA sub and top) and the sound is only amplified though the tweeter and mid driver. So its a high frequency distortion that it amplified by low frequencies? Confused 🙂