I own a '93 400E and I had the same problem. I had it checked out at the MB dealer and they said it was a power steering fluid leak. They replaced a hose and the noise went away. It cost me between 200-250 total.
On the life of a 400E, I have very high hopes. I am at about 124,000 and I have no problems. I expect to get at least 200,000 before I would consider a new car. There are plenty of high mileage 124s out there.