| A hammer and some galvanized nails should do the trick...
Actually, this happened to me the other day, although I'd be pretty sure this isn't your case. It turned out the excess length of 2 of my strings were hitting each other and it was causing some rattling. If it's not that (which it probably isn't, but hey, you never know...), it's something you most likely want to leave to a luthier to fix or take a look at, IMO. | 
