I think it was mud and water through my leaf spring and around my brakes. It hasn't made that noise since.
back when my front driveshaft was broke in half, lol http://www.youtube.com/user/THEPRODJECT01#p/u/17/bgp9aLYI2bk
That's almost identical to the sound you get when you wedge a rock in between a brake disc and a rock guard. Could have been that, but you got lucky and the rock fell out?
Wow looks like fun....but something ain't right. And what/why do you have gigantic mud flaps on your bumper??