What Sound Should The New Hippie Cars Make?

“[I]n a few years, the government will require electric cars and gasoline-electric hybrids to emit some type of noise at low speeds, when their battery-driven motors usually run silent. The promised rules — aimed at making the vehicles safer for vision-impaired pedestrians and others who rely on aural cues — have launched auto makers on a quest for the perfect sound. Among those considered: noises reminiscent of jet engines, bells, birds, flying saucers and revved-up sports cars.”
 — They probably haven’t considered this because it seems so obvious, by why don’t they just go with a smug, NPR-type voice that repeats, “Look how virtuous I am” over and over?