Any thoughts on what could be causing an annoying rattle coming from behind the trim of the rear seat belt retractor, sounds like a marble/ball bearing rolling . How can i remove the trim maybe to take a peek...sods law its 2 weeks out of factory warranty, and my Ford Gold warranty people say as the belt functions ok , ,anything else is not covered 😤( 2019 Ecosport)

Annoying rattles come and go, I can't stand them! Had plenty over the years. Don't be blaming me if you can't refit the part. But if you pull the rubber seal on the door away. You can then probably prise or pull at the grey roof liner to get behind. Choose a soft filling of your prefence and pop back into place. 

Given the description I'd wonder about a nut, back in the day when I messed with cars I have had a nut escape twice, sometimes the welded nuts can snap free when over tightened (the ones spot welded in the factory to receive a bolt later), you wouldn't expect them to come off, but like all things in life the odd few weak ones make it through the production line, or the more common one is non fixed items, I say items because it could be a nut, but most of the times it's a simple clip that slides onto the skin of a seam, that takes a self tapping screw, typical of speakers and\or trim etc.

But if it's in the cavity of the framework you'll just have to live with it, not unless you want to cut into the car frame which will weaken it and open the door for rust to get in, which is just crazy to do, but the brain just came up with a possibility, but have no idea if it would be manageable, because it's been so many years since I last knew of it, and I don't remember if cars have access points anymore (if they do it opens up the possibility of an inspection camera if your lucky).

But using Waxoyl, the last time I got some you could get an attachment so you could inject it into the cavity's of the car, the point is that 'IF' you can access the point where it is, you could inject this in, and by it's nature as you drive and the thing starts rolling about, it will become 'stuck' in the Waxoyl, well not stuck permanently, but like wading through a bog, it gets heavy going, and as such it should stop the annoying issue, or at the least seriously reduce it, but ultimately it depends on if there are access points these days, hope it helps.