So the metallic vibration/rattle sound from behind the right rear seat area turned out to be a DC/DC module under the high voltage battery. According to the technician, it's the coin fins on the module making the noise. On very smooth roads, there was no noise, but on any other (most) pavement, the sound was always present. Not terribly loud, but noticeable.
I only brought it in because the check engine light would not go off. The CEL was a new thing, the noise had been there almost since we first got the car. But since it wasn't too awful, I just lived with it as a slight annoyance. Fortunately, unlike what happens often, the tech heard the sound when he took it out for a test drive.
Apparently it was a big production just getting to the module and isolating it as the source of the noise. Then they had to contact Ford, run some tests, and now they have the go ahead to replace the module. 7-10 days estimated time for arrival of part, then probably another full day to install and put everything back together. In rental car in the meantime.
Hoping all the parts go back together good as new when they finally get it repaired. So far the service has been courteous, helpful, and professional.
So, if you hear a slight, kind of tinny vibration/rattle (hard to describe precisely) from behind the back seat most of the time, it could be more than just something that's a little loose. Might be good to have it checked out.
Will update when part gets replaced.