rattling noise from steering wheel
 

1992 Mercedes 300D 2.5 turbodiesel.

When I hit the steering wheel with hand while the car is at rest, I hear rattling sound.

I also hear the rattling sound from steering wheel - with some rattling sound from elsewhere inside the car (that's OK for such an old car) -, when I drive on poor road.

Noise itself if bearable, but I wonder whether it has something to do with safety.


P.S.:
The car went through 15000 mile maintenance at an independent mechanic recently (he did not rotate tires or tell me about tires, although tire inspection and rotation (if needed) is in the 15000 mile maintenance list and the front tires showed clear shoulder wear - left front tire a little more severe than right front tire -), but he did not tell me that something was wrong with the steering.