I just cleaned the pot as Dave suggested and... voila!... the noise is essentially gone. When I turn the knob, I can only hear a very silent scratching noise. Thanks Dave!
Of course, I could/should have come up with this idea myself.

But... after my father surprisingly passed away in spring 2004, I suddely lost all interest in the audio hobby although I continued using the old equipment. I guess, I should have said goodbye or something. ::)