It's possible that the entire side of the greenhouse is embedded within the turbulence of the side-view mirror.
This illustration gives a sense of the magnitude of a mirrors turbulent wake as a function of fineness ratio (the length divided by the width ) and associated drag coefficient
The drip rail is no doubt triggering separation,and the vortex outside the glass may resonate parts of the car at some natural frequency,plus octaves of that frequency.
High end modern cars spend a lot of time in anechoic chambers to fine tune the greenhouse area for low noise.They're even using double-pane glass for a more quiet ride.
Maybe some painters tape and cardboard can isolate the noise source,then you can think of an aesthetic fix.
Beautiful car!!!!!!!!!!!!!!!!
