Sounds like a terrible idea, it might be sound on paper but in reality it just doesn't make any sense, the loud beep is far more recognizable for me than some hissing white noise, heck there are trucks where i live that has a recorded human voice saying (careful, car is reversing) that works well  too, i'm sorry but not everything works because of a math equation or a graph, specially when there is a human element involved, i don't mean to diss science or engineering but what's the point of a graph or study saying the white noise is better if actual humans it's designed for don't feel that way .. there are plenty of other factors than just science here including familiarity with the very recognizable beep sound or the recorded human voice.