The Pedestrian Safety Enhancement Act of 2010 had NHTSA studying sound generators for EV's and Hybrids (when in EV mode) to alert pedestrians at low vehicle speeds.
The mandate was pushed back front 2014 to 2018 (I believe), but a few carmakers like Toyota have already implemented the sound generators on some cars, ahead of the mandate.
The sound was initially described to us as "whales mating"...and having heard it on a Lexus CT, I can tell you the description is accurate.
You can google it, ror read the text here:
Clicky for Pedestrian Safety Enhancement Act of 2010