I had the same issue. At 45+ MPH the passenger mirror appeared to be whistling.
When I closed the power mirrors it would go away. My Ford dealer (Tommie Vaugh in Houston, Texas) replaced the mirror assembly under warranty. Unfortunately, there was no change in the whistle.
We then began taping all the seams and openings on the passenger mirror assembly with no change in the whistle loudness or pitch. The noise would still go away when we would motor the mirror closed.
We then expanded our search and began taping the upper door seams near the mirror. The sound then disappeared at all speeds. Attached is a picture of my Service Rep (Paula) when we were working thru the troubleshooting process.
View attachment 8231
Hats off to her for not settling for the “this is normal” diagnosis. As you can see in this picture, the tape is now only on the door seam. The service team was then able to narrow down to an 8" section on the seam (near Paula’s left hand) where the sound was coming in.
Upon inspection, there was no defect found in the gasket (no cuts, breaks, cracks). Its just not sealing as tight as it should.
We believe that closing of the mirror was giving us a false positive and pointing us to a mirror issue. When the mirror was closed, I imagine the airflow was changed just enough to not cause the whistling.
We then came up with a fix that was satisfactory to me. However, others might not like it.
A solution we arrived at was to install a small piece of black felt tape on the inside of the door jam to better seal the gasket. This works on my car (black felt tape on the inside door jamb of a back car).
Again, this might not be acceptable for anyone other than me. But wanted to share it.