I'm fixing a jazz bass for a friend of mine and what I'd originally thought was a grounding issue turns out not to be.

He's just put a new bridge on it and hadn't run w wire to the bridge so I've now attached one, but it's not actually solved the problem. The wiring looks fine to me, but we have an interesting issue where there is a humming noise when anything metal IS being touched, rather than when it isn't like a normal grounding problem.

When the bass is played, it's making is sound very fuzzy and a little distorted on all settings. I'd like to work out why it's doing it so I can replicate it actually, but if I can get rid of it then that's all that he actually needs. Any ideas?