I just picked up a used John East P Retro preamp. I finished installing it in one of my P Basses, the tone controls seem to be functioning properly, however, there is a constant clicking sound.... Like a metronome at 100 bpm. It's louder in active mode, but it is audible in passive too. I already took it all apart and re-did the install... Same thing both times. Any idea what this is... How to fix?