I covered my top mirror bracket holes with black Gorilla tape cut to match the contours. Been on for thousands of miles and numerous washings with no peeling. Stopped all the whistling caused by air passing over the holes, sort of like you would blow into a pan flute.
I also put plastic grommets I got from Lowe's in the lower bracket holes, cleans up the look and would prevent noise there too if they were inclined to "sing".
Also make sure the shifter cable is not adding to the cab noise, there was a TSB on that noise Freightliner had to do to my 2014 to keep it quiet. Its noise was RPM-related, kick it out of overdrive and it would go away. It was easy to tell it wasn't wind noise, but drivetrain in origin.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST