OK, did another road test yesterday. Dry pavement, long straight, older asphalt. In 4th lock-up, 2000 RPM cruise, applied light throttle. Could not duplicate the "low moan" regardless of a/c or fan settings. Had a friend with me who couldn't hear it either. I'm beginning to think it might have been a tire noise variation caused by the fresh (days-old) asphalt we were on on Friday.
Peva, I double-checked the belt tension and it is within your recommended 1/4 to 3/8 inch slack. So I don't think I'm gonna worry too much more at this point, except to ask if such a sound could be transmission-related? (Trans works fine a.f.a.i.k.)