Glad I’m not the only one, Gavin. Do you think it’s between the window and the plastic? To me it sounds like there’s a piece of plastic below it which it is rattle against when I knock on it.

 

 

Ha! I bet my neighbor thinks I’ve gone insane with how much I was banging on that thing last night in the driveway. I shoved some felt tape up top to no avail. Now I’m thinking the issue is from below.

 

Tried shoving a card between the headboard and plastic trim to no avail. Still rattles loud as a mother. Fairly certain the black safari window trim is the culprit though, produces the same vibration/rattle when I bang on it.

 

I have 23 defender 110 with 3rd row too, plus roof rack and ladder, I have the driver side rear seat rattling from day one.
After dealer's first fixing, they claimed they just adjusted the seat, it got much better, but when the temp got warmer, rattling will come back, and I can reproduce nosie by shaking the rear seat backrest, otherwise then weather is nice and cool, no rattling whatsoever.
Long story short, they replaced the driver side rear seat frame yesterday, but it's getting cold here, so it wasn't really rattling, so we will see how it goes when weather gets warmer.