I had the *identical* sound coming from my 350SDL. I thought it was the vacuum pump or timing chain. Spend hours trying to figure it out.

The problem ended up being the bearing in the belt tensioner arm. Several of the balls had fallen out, and that sound was the tensioner arm knocking against the timing cover.

The shock should not get burning hot - but they tend to. I had 2 shocks get so hot on my SDL that they'd puke their oil out.

I'd highly recommend you install a clutched pulley on your alternator. Get the one specified for an OM606. I installed one on my 350 (along with a larger alternator) and the difference is dramatic.

https://www.youtube.com/watch?v=DGI9XLuhaYY

EDIT: Found an old video I'd taken which shows the sound: https://www.youtube.com/watch?v=puFCqJbIk3k

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.