This was how I fixed the whistling sound. I had a roll of black insulation tape from the Home Depot, $8 for a big roll, laying around in my garage.
Cut a very thin strip of it, about 2 inches long. Use good care, remove the paper backing, tug it in the gap at the corner of the mirror as shown in the above pictures.
Use one finger to rub on the surface to remove the excess until the strip is very thin and hardly noticeable. Clean any thing that gets on the painted area by rubbing it off gently. Afterwards no one will ever be able to tell you filled the gap with anything.
All you need is a bit of fill in the gap at that corner to eliminate the wind noise.