Point of clarification...
When I said "there's no noise generated simply from turning the wheel through its range of motion," that's true as long as the wheel is turned smoothly as you would in normal use, where the angle of rotation is fairly significant.
However, if the wheel is turned somewhat rapidly back and forth ("jiggled") over a short range of travel, you *can* feel and hear this rattling sound, and it is definitely coming from the steering column. I did this with the car sitting still, in P. Kind of lines up with the observation that gravel roads cause the rattle, because the less-smooth surfaces probably cause very slight angle changes from the tires up through the column as the vehicle goes down the road.
The symptoms do seem similar to the MDPS (Motor-Driven Power Steering) Coupler problems seen in Kia / Hyundai designs where the "soft" star-shaped coupler gets all chewed up and lets the metal parts bang into each other:
https://www.youtube.com/watch?v=rxTcFnykutc
https://www.youtube.com/watch?v=1YS94TPb8Us
https://www.youtube.com/watch?v=Y5BnVlfSVug
I don't know yet whether this coupler (or something similar) was still being used by the time the 2016 Sorentos were built.