Isn't the womb a fairly noisy place, anyway? From the perspective of a fetus, that is. I remember when our twins were born, white noise was recommended to us by practically everyone because of the similarity to sounds heard in utero.
Odd, I would expect a steady beat to mimic your heart to be a big part of mimicking sounds of the womb. Along with a colored noise. Though, I would suspect pink is better than white.
They're essentially under water and a pregnant mothers noisy internal processes, other than her breathing and her heart beat, are not really following any sort of pattern. And we can't discount the environment that the pregnant mother finds herself in, either, since that noise will also be present to varying degrees.
edit/ Having said that, the "white noise" machines that I've seen, which are available as sleep aides specifically for young children, are not really white noise. There is white noise there but there is also something that sounds like a tumbling drying machine, or a fan, or a hair dryer... Something that a brain could attach itself to as a pattern.
