I've never heard of a truck using white noise instead of a beep, but that might just be working as intended. Here's a video of what it sounds like: https://www.youtube.com/watch?v=6rwJ5NCf1Vw
I think I may have heard that noise, and just not known what it was. It could easily be mistaken for some sort of mechanical bit of the truck (slipping belt, various bits and bobs going around in the engine).
