The driver side mirror whistles above 35 mph and was driving me nuts.
I was able to determine that the ridges on the surface of the outer mirror rubber seal were whistling in the wind. I tried wrapping black electrical tape around the mirror and, while it did fix the whistle, the tape didn't last long. It is the center groove in the rubber seal around the mirror that causes the whistling.
My solution: Wrap a length of black paracord tightly in the center groove all the way around and mark. Cut the cord and melt the ends together (heat both ends at the same time with a lighter and push together). This eliminates the need for a knot and allows the cord to sit flush in the groove all the way around. Then stretch the loop of paracord back around the seal; it should fit tight. This may take some trial and error to get the length right.
Fixed the whistle, has lasted over 2,000 miles at highway speeds, and is virtually invisible.
Hope this helps,