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Mystery Knock on 80q

My '88 80q now has me a bit worried.  I have wrenched on all sorts of
vehicles for years, but I can't figure out the source of a engine knock.

Starting a couple of years ago, on cold mornings, the engine would have a
very metallic sounding knock, that would go away totally in about 1 minute.
 It wouldn't ever happen above 60 degrees or so.  (This has nothing to do
with lifters; doesn't sound like the typical Audi lifter click sound.  I
replaced all the lifters to get rid of this problem) Very intermittent, so
I didn't do anything about it

Now I moved to California, but the knock has appeared again, only
regularly.  Like the earlier knock, it appears to be temperature sensitive.
 Additionally, it seems the exhaust is rattling a bit.  It almost sounds
like a rod knock, but it is very condition sensitive:

1. Doesn't knock at idle. (exhaust rattles & is a bit loud from a leak,
2. Doesn't knock if you rev the engine while parked.
3. Only hear it under load; accelerating, usually between 1700 and 2500 rpm
4. Very pronounced/loud when engine cold. 
5. When engine is very hot, can't hear it at all.

I have never heard a knock that is this condition-sensitive.  Engine runs
great otherwise; plus it only has 180K miles!

Can anyone shed any light on this before it goes catastrophic?



Knocking '88 80q