From an older thread. Rear seat rattle can be difficult to locate (#3 below)
A few possibles to try:
1. Give the (cold!) exhaust a wiggle, tail pipes first and then centre section. If a mounting rubber has gone missing the exhaust can move, if it contacts an undersealed area you can get a dull knocking.
2. Rear shocks debris shield. Look over the top of the rear wheels, there's a plastic tube which covers the exposed shaft of the shock, it locates into an aluminium fitting at the upper mounting point. The plastic can separate from the ally fitting and again gives a dull rattle/knock.
3. Rear seat catches. There needs to be a reasonable amount of "pre-load" on the seat retaining latch, if not you can get movement and... knocking.
Try driving with the rear seats folded flat to diagnose. As a temporary measure wrap some insulating tape around the metal loop of the latch, this should increase the pre-load whilst stopping direct contact of the 2 parts responsible for any noise.
4. Still no joy? Rear hatch rattle similar to the seat issue above, try the same "tape fix" as above to aid diagnosis.
Full thread
HERE.