I just wonder, why does it have to be engine noise in particular? If I saw a Tesla, Leaf, Volt, etc. I wouldn't expect any engine or exhaust noise. Beeping, maybe.
Benny Hill theme, played at a speed that matches the car’s speed.
I'd prefer La Cucaracha
I expect there'll be a new set of businesses built up around custom engine noises. If the manufactures don't make it easy to load your own MP3, then you'll probably be able to take it to a car mod specialist.
Beeping? Like a lorry backing up? Even a low pitch beeping would become annoying pretty quickly. It should just be a sort of hum.
A hum would work just as well. I was just trying to think of a noise I would expect from an electric device, and beeping was the first thing that came to mind. It's better than coil whine at least.