Update: well, after making the video shared earlier, it has never made the sound again. didn't occur on way to shop, shop couldn't reproduce, got it back, have driven it for several weeks including several multi-hour drives .... nada. maybe it really was something kicked up off the road, or perhaps one of those lost 10mm sockets managed to free itself and got kicked around by the fan blades and finally knocked out. it's a mystery, just like the number of licks it takes to get to the center of a tootsie roll tootsie pop.
got the truck serviced, trans oil flushed, yadda yadda. running great. will continue monitoring for noise...
