From the Canon Canon Digital Learning Center:
"You just took your Canon Image Stabilized lens off the camera and quickly replaced it with another. But as you put it into your camera bag, or later when you take it out, it seems like there’s a soft rattling sound coming from inside the lens. You give it a gentle shake and sure enough, something’s loose in there. Is your next move a call to Canon’s service department? Not necessarily! We’ll describe why in this article."See the entire article on the Canon Digital Learning Center.