Ok, I've got a 1980 Washburn Force ABT B-10 when I pull active knob. Not there when passive. I get a hissing sound that is there always. Whether I'm playing or just sitting there its fairly loud. Not so loud that you can hear it over playing. It's there whether I am touching strings or bridge. I'm not sure what it might be. Possibly a shielding issue? Any ideas as where to start?