I recently bought the Canon 100-400 IS primarily for sports photography and am slighlty disappointed that it has a certain "clunkiness" during focussing. I have read elsewhere that if using a tripod or monopod (in my case) the IS should be turned off. Is this why it is clunky? I am on the point of returning it to a dealer for checking it out. I can't see any advice about this in the manual.

