It could be a number of things, obviously.
I would have a friend ride in the back while you drive or vice versa and see if you can at least pinpoint which end of the car it's coming from. Without that starting point, I think you'll be chasing your tail(no pun intended
).
BTW-I absolutely cannot stand rattles,creaks or other sounds in my vehicles. I will periodically go through and tighten all of the fasteners I can locate in the interior. I have a "rolling coin" sound in my Mazda that has me baffled. I have literally taken the seats out, doors and most of the dash area apart to try to find it but cannot.