weird, i had the same thing and could not find it, with me it was on the driver side, emptied out the trunk, would drive me crazy, finally put my mechanic in the trunk and went out driving and he figured out it was the top of the schock, it needs tightning, but you need to remove yr speaker to get there, it just loosened over time, its gone now....
let me know if it worked..
good luck
frank