I had a sound very much like that develop on the way to Rubithon. It was more periodic, totally speed dependent and was driveshaft related. Was not tough to fix (pocket knife) but was hard to diagnose until I got enough coffee on board to wake up.
It's probably not what you got there, though. I'd be going after brakes, CV, hubs. That sounds to me like the auto hubs on my old Ford Ranger engaging and disengaging. They'd pop to lock and pop when you backed up to unlock.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein