First thing to do is change the strings. I there is still a problem then look at the guitar set up (string hight and neck curvature).

Between these two you should sort the problem out. If not then it is time to look for things the string may be vibrating against (i.e. a bit of pickup, a broken nut, check the neck isn't warped, a troublesome fret that needs replacing).

I wrote a lesson on guitar set up that may be a good starting point, it is in the studio section.

Hope this helps some