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Hybrid and electric vehicles are potentially silent killers thanks to their stealthy electric engines that do not warn pedestrians they are coming. So goes the argument for making them produce some kind of warning noise, a proposal strongly backed by the National Federation for the Blind.A bill backed by 16 US members of Congress would require the Department of Transportation to establish minimum sound levels for all hybrid and electric vehicles. And just last month, a mother whose son was hit by a Toyota Prius raised concerns over the stealthy nature of such cars.Two Stanford students think they have the answer - you can see it at work in the video above.It makes a noise that to my ears sounds a bit like a throaty motorbike in need of a service.I blogged previously about the vexed question of what noise the cars should make – reader suggestions ranged from Stars Wars noises to the sound of horse's hooves.Do you think this Stanford system is any better than those?Tom Simonite, online technology editor