Hi guys, I am really struggling to figure out a weird slapping/tin can knocking sound in my 82' 300D.

I've spent a LOT of time poking around with a stethoscope and it really sounds like the noise coming from the top of the valve cover by the chain. Here's what it sounds like while running diesel purge:

https://youtu.be/G1TOgGyp81Q

I have done a valve adjustment, I have ran diesel purge a few times, and I have shut off each cylinder and the noise remains, so I don't think it's my injectors.

The car runs excellent. Virtually no smoke, little oil consumption, smooth idle, lots of power, average blowby.

I have pulled all the accessories off the front, inspected the vacuum pump, and the shaft that attaches to the pump, and everything there looks great.

I checked chain stretch using the 'line up' method (don't have a dial right now) and it's right at 5 degrees.

Adding to that, there's another thing I've noticed. Upon startup, it's quite clattery, but then after a few seconds it quiets down a lot, then slowly comes back. This video captures what I'm talking about:

https://youtu.be/mWvWSlQsphE

So, what do you all think? Timing chain tensioner? Oil too thick? (20w50 in San Diego) What else?

Sorry for the long post, I am really stumped.