For a while my Sigma 18-50 f/2.8-4.5 lens has been shaking the viewfinder image once in a while. After asking a related question I settled for the explanation that it was the image stabilisation kicking in. However, now I noticed that the shaking happens only when I zoom in/out around the 24mm focal length. The image suddenly jolts when the lens passes the 24mm.
Looking at the lens from the front I see that the actual lens element jolts when I pass the 24mm mark. Turning the camera off and zooming in/out does not yield the same behaviour. Taking the lens off the camera and zooming in/out also works fine. The AF/MF and IS switches have not effect on the behaviour. Currently I can perfectly work around the issue, but if this is a symptom of some kind it would be good to know.
My question is: what am I seeing and might this be the beginning of the end of my lens?