grounding shielding

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...

Have an SX Vintage Style p/bass and it has a ground running from the tone pot to the bridge ,, I' shielded all the cavities, still getting a buzz without touching the bridge or strings etc.... should or can I run another ground from the input jack to the bridge?