This is sound design applied to truck engineering! White noise has a flat frequency spectrum," which means that it has equal power at any frequency (<a href=""http://en.wikipedia.org/wiki/Colors_of_noise#mediaviewer/File:White_noise_spectrum.svg"">http://en.wikipedia.org/wiki/Colors_of_noise#mediaviewer/File:White_noise_spectrum.svg</a>) so it can be perceived even when other sounds or noises cover some of the frequencies", and I believe that boosting the volume of all the frequencies (white noise) will be perceived as having a much greater effect than boosting the volume of a small frequency band (beeping sound).