Thanks all!!
I'm reasonably confident that (1) I have the later style vacuum pump and (2) the noise is caused by a combination of worn parts in the belt idler/tensioner assembly.
Here's the pump that's on the car:
It is the later style pump, right?
The idler dampner has loose ends (the Heim joints are worn) and the idler pully is running at a slight angle in relation to the belt. The belt looks OK so far.
Today, I'll order all the parts needed to replace the idler assembly.