If the string sounds off NOT plugged in, it's the string (doubtful). If it sounds off only when plugged in, it's probably got to do with the pickup (known to be trouble).
If the second is the case, I would loosen the strings and see if you can wiggle the saddle around then "seat" it by pushing down as you tighten the strings again. If that doesn't do it, could be a lot of things. All of them kind of a pain in your butt. Do you know if the PO modified it in any way?