I had Tce100 Gordini that had a rattle in the back, thought it was the parcel shelf whackering at first, but eventually discovered it was the rear seats ! In order to lift up and fold over the rear seat( after the back rest is laid flat) a spring loaded locking pin is dis- engaged by the red levers on the on the rail of each rear seat from the boot of the car. The spring loaded pin locks into a hole in the body shell of the car, this can be seen when the seats are lifted as a piece of carpet is cut out around the hole. The locking mechanism is not a 'solid lock' fit in the hole when the rear seats are in the normal position, therefore ther is about 1-2mm of play - hence a rattle (sounding like it's the parcel shelf) when you go over a bump.
The cure- I have recently bought an RS 133 with Cup chassis and experienced the same rattle problem. This was cured by making a set of washers for each rear seat from an old bike inner tube. About a 30mm dia with a 15mm hole ( only approx) anything thicker than a couple of mm will not work as the lock down mech on the rear seats will fail to engage. On my TCE100 I used duct tape originally - this worked for a while but eventually the metal against metal wore through.
My car runs solid and without anymore annoying knocking/rattling.
Hope this helps