Oh, er, which involves

levering out the red tab in the plastic door pin with a flat blade screw driver, if you don't want to take the door card off. Although this will mark the red plastic tab slightly. And applying some sort of sticky stuff to the metal rod, to keep it still.sticky stuff can be double sided tape, blue tack, impact adhesive or whatever you have.
There's two elements that can rattle.
Without removing door card...
The metal rod in the plastic pin. (Remove red tab, apply sticky stuff)
The plastic pin in the plastic door surround in the door card. (Apply insulation tape, which didn't work for me)
But to do it properly
Remove door card.
The metal rod rattling in the plastic pin.
Unscrew plastic pin from metal rod, remove red tab without damage. Apply sticky stuff.
The plastic pin rattling in the plastic door surround in the door card.
Remove plastic surround in door card by pulling the collar off the back. Replace felt. Re fit.
For good measure, but not really necessary of doing both options above, bend the metal rod the plastic pin sits on so the top of the plastic pin leans back towards the window. It then sits at an angle and takes any play out of the surround and on the metal rod.
Take your pick.
