So... I decided to try an experiment. I started the car and attached my
Mity-vac hand vacuum pump to the EGR valve and gave a squeeze. Soon as
the valve opened, the engine quieted down (the horrid clacking quit) and
the engine ran a bit smoother, not worse as predicted. Now... does this
mean my engine is running too advanced (this was only at idle, of
course) or does it mean something else? At present I can't check EGR
effect on the road, but everyone I've spoken with thinks the symptoms
match EGR "engagement". Not sure what else would cause this effect,
because power and starting seem just fine. There's no "missing" as
there would be in a spark-ignited car.
Bad injector maybe??