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Mystery Knock (partially solved! update)

Hi all.  Thanks to the many folks that responded with clues.  

Last night I took off the aux vacuum pump, removed the push rod &

MOST of the knock is now gone!  However, I still have a rattling type of
knock with the exhaust.  I put it on jackstands in the front, blocked the
rears (mandatory safety note) & crawled under while running.  I couldn't
really find where the rattling was coming from.  Seemed the cat was where
most the noise was from, but it was elusive to pinpoint.  (however the thin
tin shield around the manifold was rusted thru all over and was loose; my
thin gloves prevented me from reaching up while it was running to investigate)

What I found unusual about this knock scenario, is that the aux pump knock
and the "other" knock (still unsolved) were combining to produce a very
loud metallic knock.  I'm halfway solved but still need more investigation


Quieter '88 80q