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
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