Hang on to that seat belt!
The 200 series LC has a little clip to slide that metal seat belt connector into so it doesn’t bounce around and make noise at the back seats.
In the GX, I just keep my back seat belts hooked up all the time, and don’t let the seatbelt withdraw at the front seats...
I have one spot like that on my drivers side door. Was there when I took delivery.
My theory is it’s the seatbelt metal knocking against the door when the seatbelt is unbuckled and withdraws, lightly scraping it.