To cure this you need to purchase some sticky felt and apply it to the bottom edges of the door panels where the hard plastic door panel "edge" meets the hard plastic door sill plates. There is also a spot on the front doors that can creak. Pretty much impossible to explain that location here on the forum. You can cure 99% of your problems by doing the felt on the door panel trick first.

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1984 300 SD 384,000 Miles

This is not the characteristic scruffy sound we all get with 126 cars from the plastic cladding rubbing on something in or around the door jamb. All that went away years ago with regular applications of silicone spray. This is a hard chirp that only occurs on right hand curves at road speed and only in the area of the right rear door,.....actually thought it was coming from the rear seat........( and it may be)

Im willing to try the felt thing, but, I would ask you if you tried a thorough dousing of silicone on all those edges before you tried the felt thing and I would also ask if you had a very loud and pronounced singular "CHIRP,CHIRP, CHIRP, CHIRP....................?

Sounds like something different. No idea what it could be... I went straight to felt, never tried the silicone so I can't comment on whether the felt reduces noise further. Are you sure the sound is coming from the interior?

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1984 300 SD 384,000 Miles

I tried both ;only the felt worked and still very much effective.
Rd

How much sticky felt are you folks talking about? Is it like 2 small squares/circles? Or does a bead of it need to be applied?

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
No longer selling Cluster Needles Paint
No longer selling New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more

Yeah, might be different than the squeak of the lower door panel on the sill molding cover panel.
Not possibly a wheel bearing - ouch!
Might be the seat mounting where it snaps down; might pull it up and lube that.
I had good luck on the usual squeaking area by lifting the little rubber flap on the sill cover and applying silicon grease under that lip. Its a little vertical flap about 1/4 inch tall that contacts the bottom of the door panel when closed, it gets squashed up against the sill panel, and may itch against it, creating the squeak.

DG