Whenever am driving along the motorway doing above 68ish, there is a whistling noise coming from my passenger wing mirror area.

Does anyone know why? Or have any tips on what I can do?

I've studied it and the chrome cap does move slightly....but I can't see why it is whistling. It never used to.

Thank you all!

Has a clip broken that holds the mirror cap?

Hi ya

You need to find the problem first.
I would suggest that if you think it's the mirror cover. you should tape the gaps around the cover and mirror. Use something like masking tape, then go for a drive see what happens.

I have a similar problem but its the lower windscreen chrome trim after the screen ad been changed and the trim was not refitted properly.

Hope this helps