Sorry for the misleading trail - the problem did indeed turn out to be the problem that "bdespain" described, "the 2nd row outer seat belt clips tend to fall between the seat and the door and rattle on the plastic seat trim."
Thought I had eliminated it, but was then fooled later. The sound travels so that it sounds like it's coming from behind the drivers left ear, when in fact it's originating much lower.
You have to carefully turn the (outer rear) belt tangs the proper way when getting out of them so they don't hit the door pillar plastic.