Hm. Maybe, it's the bridge retaining wire that's resonating? Due to a bad screw hole in one of the saddles one of the screws in the bridge of my Epi LP raised the wire above the screw and bent it a bit. As long as I remember, it hasn't caused any problems yet, but your rattling can come exactly due to this kind of issue.
EDIT: Oops, couple of seconds too late :)
EDIT2: From your current pictures it looks to me that in fact the b-string screw is higher than the others.