It does not happen idling in Park or Neutral, it might
happen parked in Drive - say, at a stop sign, briefly.
First I ratchet strapped the heat shield to the muffler - this did not help.
Then I started focusing on different areas. This bar extending from the spare tire up to the bottom of the rear door was "loose" and it seemed a good candidate for rattling - this did not help.
There was also a ton of gravel in the rear bumper - this did not help, but made me dirty.
Then I found that this "bracket"-y type thing around the left-rear brake would rattle if I moved it. It was not easy to move but struck me as something that would act like a tuning fork, so I stuffed some shop towel in there - this did not help.
Around the same place, there was a great many number of rocks between the gas tank and its plastic shield. I did not resolve this, but I suspect it is not the issue.
I took this video to know what it sounded like outside the van. I was also curious if the drive shaft rotated any other time than moving (I was considering grinding diff gears at this point)
I could reliably reproduce it at a stand-still in Drive, with the emergency brake on, barely
pushing the accelerator (which is what I'm doing in that video^).
! there was a woman in another van in the university parking lot near me, so I asked her if she would help me by revving the engine slightly while I was underneath it. I agreed that the windowless white van and overalls were indeed conspicuous and assured her I wasn't Ted Bundy nor Buffalo Bill - and she did.
I put my hand on the muffler (can, as I have called it) and :bouncy: it rattled hard along with the sound, and only when the sound could be heard.
So, I think there is almost certainly something inside of it that is loose or from upstream. I am not sure if this is a satisfactory conclusion, yet
, but I am happy it is rattling and not grinding.