I have had this on the 16v which Im pretty sure has the same rear suspension set up as your car. Check all the rear suspension rubber pieces and especially the trunion joint which is on the outside of the lower control arm. It sounds to me like one of the rubber pieces has wore out and is causing this noise. Also have you checked the tightness of the rear shock top mount nuts?

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2006 E320 CDI
1998 S320
1987 190E 2.3-16

1990 300D 2.5 Turbo(sold)
1987 300SDL Turbo(sold)
1987 300TD Turbo(sold)
1987 190D 2.5 Turbo 5sp(sold)
1985 190E 2.3-16(sold)
1984 500SEC(sold)
1984 230TE(sold)
1978 240D(sold)-Miss it!
1972 280SE 4.5(sold)