The only thing I can think of is to push some anti-rattle tape in the gap between windscreen and dashboard temporarily- don’t remove the sticky backing and leave a long length showing so it can be pulled out and done tidily somehow if that cures the
problem.
If that does work, then you can figure out a way of doing it tidily.
Otherwise, you’ll have to figure out
how to remove the air vent. I gave mine a little pull at the corner a few days ago to see it would unclip easily and it didn’t want to unclip, so I didn’t force it. You also don’t want to risk something snapping off and falling down the vent as I doubt you’d ever get it out…
Hopefully somebody else will come along with a better suggestion than mine, and
how to remove that air vent.
Cheers,
Nick