Just had those creaking noises happen last week, same place. Slow turning, speed humps ... creak groan squeak. Loud. Embarrassing.

For diagnosis, using a needle on my grease gun I got some grease into various bushings. Noise quietened, but was not gone.

Raised car on stand, so the wheel was just off the ground. Placed trolley jack under control arm and raised it slightly (it's under tension from the spring). Inserted pry bar and checked for play in balljoint. There was about 5-10mm movement, and there should be none. Got some grease in there. Noise disappeared.

I was due to take family on a long drive later that afternoon. Swapped to small Toyota for that trip, as I didn't want to run the risk of balljoint separating at speed , in traffic , out of cellphone range , where the best possible outcome is to get a flatbed to recover vehicle to place of repair way the hell somewhere else etc.

There are threads on replacing balljoints, and my Haynes manual has a straightforward account, but I don't have time, or a press, so took car to the local Euro indie: 3 hours' labour plus parts and a wheel alignment. They compressed the springs, took the control arms off, pressed the balljoints in with an adapted hydraulic press, replaced, then took it for an alignment.

Result: no noise, and that old Merc steering feeling has come back, responsive, accurate, but not insistent.

FYI:

The Wurth application I described earlier has begun to dry up so I had to get more aggressive. For great results I recommend buying one of these, in stainless steel:

Flavor Injectors
http://www.bedbathandbeyond.com/search/search.aspx/injector/?sstr=injector&grid=20&dim=1&nty=1&

Then:
1. Suck up some moly-lithium grease into the syringe (the vacuum is probably strong enough to suck up peanut butter)
2. Screw on the steel needle
3. Inject grease into ball joint boot.
4. Creak is gone -- hopefully for a long time.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black