Update: The Mercedes Benz service folks (at Istate, Blaine MN) looked at it very thoroughly and checked everything under the unit to make sure it was all secure and nothing was broken. They drove it twice and again once with me and I could not replicate the sound. We took tight corners, pushed the load a bit to shift it around, and went over some very uneven parking lot terrain - nothing amiss. So, we agreed to leave it at that. Since there was no warranty work to be done, I paid for the mechanic's time and consider it worth it for the peace of mind that nothing was broken.
Upon return to the RV storage lot, plenty of clunking sounds pulling in an getting settled. It sounds like the box sets down then tries to adjust its position a bit before coming to rest.
At this point, if something breaks I'll get it fixed. Otherwise, it's a "live with the occasional clunk" situation.
2016 Forester 2401R