Sound like a rock stuck in a metal box of some sort
every time it hits a crack, that the two rear wheels getover a crack or kind of the suspension when it travels it kinda clicks/clinks- clink - clink..

we lifted the truck, saw there was missing the plastic pucks on the blades, the rest seemed to be all tight, i took the tailgate off, still doing it.

the sound is getting annoyinh

shocks maybe?
brakes?
hand brakes?