I had a 1977 240D and it made that same annoying sound when the car was cold. However, I have to say that in the two years I drove the car, the noise never got worse. If you dont' mind the noise, I would not worry about it.
Just to make sure, the noise you hear is like a very very loud angry bee in your instrument cluster and your speedo needle starts going nuts, right?
For some reason I still have this noise once in a while in my 1992 300E when the car is really cold. But since the 240D prepared me well, I don't mind it too much.

Leo