Check:
headrests, rear seats are locking into position, theres nothing under the seats, seatbealts, anything in boot, tool kit, spare wheel well, take the rear hatch panel off and check the rear wiper mechanism, check the trailing arm suspension bushes, check the shocks, check the rear calipers, check the springs and rubber bushes, check load sensor on the axle and finnally check the exhaust mounts and make sure there is enough clearance between the pipe and the axle/underbody.
not ur number plate is it? i had that problem on my old fiesta, double sided foam strips sorted that out.