This probably isn't applicable to this specific problem, but here is something I found out that's good to know.
Several times I've heard a rattle in my Fusion that sounded like it was coming from the right rear passenger area. Each time it turned out to be a loose passenger sun visor. It's amazing how the sound gets reflected and appears to come from the right rear. So much so that I've actually checked the rear door for loose parts, even though it's happened 3 times already.
I suggest having someone sit in the rear seat to verify the location of the rattle. Acoustics can be very tricky.