My SD has a similar creak and groan. I’ve pretty much given up.

I noticed you haven’t mentioned shocks. What do you have in there right now?

My noise is probably coming from the shocks. Sometimes I spray the spring pads and sway bar bushings with silicone oil and it may quiet it down but I’m not 100% sure that’s it. Looking forward to hearing what you find.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles