Neuro, thanks for your reply.
From your questions and explanation I have somehow the impression that my lens behaves normal. Just because I don't have this on my 70-300L I was worried.
Here is my little test:
When I half-press the shutter, the IS motor is starting. It makes a sound like a scratch - so less noisy than a clunck, The scratch is followed by a gentle sound of a circular saw. When I release the shutter again I hear the scratch.
When I turn off the IS, no sound is made.
I guess everything ok?