It may be worth the time to put the front end on jack stands and have someone move the steering wheel back and forth while you are under the car. I had the same noise a few months ago. From the posts I read, I was almost convinced it was ball joints. Luckily, it turned out to be the steering damper strut. I sprayed some LPS lube in between the inner and outer housing and the noise stopped. I have since replaced the strut, which only took about 15 minutes.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles