Since the rattlling began after I did the brake job, I decided to again remove the rear right wheel. Nothing odd, so I inspected and "rattle tested" the exhaust system as well -- so far so good.
Put the wheel back on and decided to tighten the 2 smaller hex bolts for the support plates on the frame floor (as the shop manual describes it). The two were a little loose.
Anyhow, went to test drive and the rattle was gone. We'll see if it stays that way.
Wonder what started that in the first place?