I get the same groaning noises from my 126 but was afraid to admit it sounds like an old Pontiac. I found a lot of the intermittant noise appeared to be coming from the spring it self and possibly the lower contol arm bushing. I haven't done any real correction other than spraying some gun oil on th spring.

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)