Interesting - I wonder if I too have the same issue.
I'm used to a nice quiet interior, but on some journeys a mysterious n/s/r rattle really gets to me. S211 in my case, but my kids reckon it's the rear seatbelt mounting area too.
It sounds nearer than that from the driver's seat, more like the centre armrest in the rear seat, but the kids (closer to the origin of the noise) are sure it's higher and slightly behind them, i.e. seatbelt mountings.
I wondered if it was something to do with the load cover mechanism on the back of the rear seats, but you saloon guys obviously won't have that, so I am now doubtful that this is the cause.