The #1 undeniable attraction of an electric vehicle from an aethetic perspective is it's quiet. They are generally spartan compared to other vehicles in their class & this is alos it's worst safety characteristic.
Horses clomp, engines roar & rev, turbines whine, locomotives chug, electric stuff wooshes you hear the air parting you might catch the whir of the motor but, it'll be too late for it to matter at 20mph or 45mph.
Sound, sound is a shockwave in the air it's an invisible force expanding from it's source in all available directions until it loses momentum & dissipates. The electric vehicle may benefit from it's need of some noise in more than one way, it may be that a sound wave projected out the nose could act as echo location for any form of autonomous vehicle & it could disrupt the wall of air in such a way as to have significant effect on aerodynamic efficiencies. A very low pitched zipper sound seems a good place to start & tying the volume to the speed seems wise as well. A shot gun microphone as a projecting speaker would make a decent prototype.