Probably to simulate dirt/mud/pebbles that stuck to the tires getting flung off. I live in a rural area and can confirm that mud and stones that got stuck in the tire tread continue getting flung off the tires making that noise well after getting back on a paved surface.It stays around for about 8 seconds I think. Bit strange that's all.
One of my bigger gripes about the sound is that helicopter flying overhead. I hate that thing, Nurburgring Carousel and Daytona Oval. In both places you are driving into a bank corner and you can hear the chopper overhead and can never be sure if it is just the copper or if the car is bottoming out a bit.
The breaking glass sound is pretty odd also but the chopper annoys me to no end and makes me think I am playing NFS or Horizon rather than Forza