Mystery engine noise

[omoore3]

I bought my 1974 300D 14 years ago from the first owner. It had 14,000 original miles. The engine started immediately, didn't smoke, had great oil pressure and would take the cart past 100 without breaking a sweat.
But it had a knocking noise coming from the front of the engine. That would go up and down with the rpm. Checked valve clearances, replaced injectors. Nothing changed. Replaced the vacuum pump. No change.
Over the years various mechanics have suggested piston slap, worn wrist pin or rod bearing and sound of the piston hitting the head.

Thinking it might be heavy carbon buildup I have used diesel purge a number of times.
I have tried 50 weight oil. No change. 50 weight with Lucas. No change.
Every few years I get curious, find a deserted road and run it over 100 for some miles. No change.
Now it has 144,000 miles and I am getting it ready to sell. This is my last attempt to isolate the noise without tearing the engine down. Any suggestions would be most appreciated.

Thanks Oliver