I am fairly new to this forum, so excuse if I might be going out on a limb.
I had the same problem with my 82 240D, when I bought it, it ran seriously rough an I tinkered with it until found this forum.
They really put emphasis on good fuel and a good additive to the fuel.
I ran "Diesel Purge" from Lubro Moly through my system and it totally changed no more knocking and ringing.
Also I got bold and just sped up the idle some, which with air on was the way to go.
She purrs like a kitten now.

Good luck, Wolfgang

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Wolfgang Marquardt
1982 240D, 270K
1983 Ford F100, 260K