Just finished a partscaster and it sounds alright, but it hums when I'm not touching the strings. The hum stops when I touch any metal part or I turn the tone knob to cut the treble. I've noticed the same thing happens with my CV, but the hum's quieter. I'm just palying through a Champion 600 RI. Can you tell waht's happening, and if there's a way to stop the hum? Thanks!