I understand both the reason for the sound alert and the distaste for carrying forward the existing bland hum. Perhaps the NHTSA could come up with an acceptable standard definition (x db, localizable, y frequency histogram, etc. ) that would guarantee the vehicle was audible, and then curate a library of tone generators that were acceptable for use on city streets.

I'm not the only one who would like his vehicle to purr like a kitten, literally.

But this is just a general instance of safety standards that try to achieve their goals by indirect means; rather than specify what exact sounds must be used. Why not have a requirement that a vehicle must emit a sound that is audible , indicative of a vehicle and not an offensive breach of the peace.