The rear seat back catch was not covered in plastic when new. To be absolutely clear I am talking about the metal part attached to the body of the car. There will be a rubber 'cover' slipped onto the catch that sits between the base of the catch and the plastic cover panel, presumably to prevent direct contact (e.g. noise) and also act as a spacer.
The rear seat back latch isn't covered in plastic either BUT there is a small rubber / rubbery type ...thing in the latch that is used as an endstop. Presumably it also prevents rattles because it prohibits front to back movement when the latch is engaged. To be absolutely clear I am talking about the latching part attached to the seat back itself. Over time and through environmental conditions this thing gets compressed, becomes crumbly and fails. When new the colour is a dark orange. Part itself is only available together with a new latch so use a suitable replacement.
Another reason for rattles can be a failed centre pivot bushing. Both sides of the rear seat share a common pivot point. The 2/3 or passenger side (if LHD) rear seat back has a metal spike going through the pivot point where it's supposed to be 'resting' on a plastic bushing. No bushing -> noise.