Daimler’s e-Smart city car hums like a real car, even though it's electric. It’s a sound-engineered engine noise, pitched higher than conventional vehicles, while Mercedes’s $569,600 SLS AMG Coupe Electric Drive has faux engine sounds to to reflect their power. Renault SA offers a choice of car tones on the Zoe hatchback, while Nissan's Leaf, the best-selling electric car, also comes with artificial sound. Regulators are looking to require warning noises as soon as next year.