In NASA's study of the silent flight of owls they mention the wings leading edge 'comb',the trailing edge 'fringe',and the downy feather sublayer which dampens turbulence.
In our modern aerocoustic wind tunnels we've got a great opportunity to tune out some noise as is happening already with side mirrors.
Also,I notice Ford SUV rear bumper fascias resonating as they go down the highway,beat up by the buffeting of the wake.If they'd run faster tunnel velocities,they'd see this and fix it before it makes it into production.
