The rattle sounds to me like it is deep in the motor but not steady. Not a rod knock and I would not think a wrist pin. It does not matter if the motor is under a load or not and it only does it after the motor oil has reached normal temp. no rattle when first started. It all most sounds like a loose timing chain, but with a spring/oil chain tensioner I dont see the rattle being intermitent any thought would be helpful
thanks

would think it is the timing chain slipper guides worn/ broken

What year 300SD is this? Are you sure the occasional noise is not in the turbo, trap oxidizer, or catalytic area?

1980 not sure of anything at this point. But the sound I am getting is internal.