The world's first purpose-built hydrogen-powered bike could be fitted with an artificial "vroom" because of worries its silence might be dangerous.A prototype of the motorbike, which could cost more than $8,300 (�4,500), was unveiled in London on Tuesday.The problem with the "fuel cell" bike, which produces no polluting emissions, is that it is too quiet.But anti-noise campaigners said they welcomed the prospect of a motorbike without the usual excruciating roar.For their part, manufacturers said the fake engine noise device, which could be switched off, would help alert road users.The motorbike, known as an Emissions Neutral Vehicle (ENV), has a top speed of 50mph (80km/h), a range of at least 100 miles (160km) and can run continuously for four hours before the fuel cell needs recharging.Its water-vapor emissions are so clean that they are drinkable, according to its designers.But with a noise emission equivalent to an everyday home computer, motorcycle enthusiasts thought the "exhilaration" factor was missing."They can add all the noise they want, it will still lack the va-va-voom serious motorcyclists look for," Jeff Stone of the British Motorcyclists Federation told the BBC.Concerns were raised that the motorcycle was too silent and might not be noticed by other traffic and pedestrians.