The control rod from the rear torsion bar to the rear axle (Item 23 in the EPC link below)
can fail and result in a clunk.

https://nemigaparts.com/cat_spares/epc/mercedes/1/fg/123123/68k/32/165/

This happened to "Desert Rose" back in 2019.
https://www.peachparts.com/shopforum/3977983-post1.html

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022