Tony,
Your problem may well be just a hydraulic tappet that wont pump up.
The classic fix is to put about 1/2 -2/4 pint of Marvel Mystery Oil in the
crankcase.     In England this is known as Red-Ex upper cylinder lubricant.

It does a great job of dissolving deposits,varnish etc that accumulate in the
engine and cause rings, tappets etc to stick.
You can pour a little down the carburettor throat too at a fast idle and it
will lubricate the top of the engine well and do a de-coke.    Do it out in
the country as it makes a lot of smoke.
If you can bear it , warm up the engine then rev it quite hard, to get the
oil pressure high.    If it gets a bit quieter, go for a good run.
This may fix it though the usual disclaimers apply.
Its not an unusual problem.   My '84 does it sometimes too.
Strangely, if I start the van and then shut it off before it warms up, it is
much more prone to be noisy than if I drive it till its hot before stopping
it.

I sincerely hope that this is your problem as the fix is cheap.

Cheers,    Steve