I recently purchased an EF 24mm f/2.8 IS USM from eBay and I'm not convinced the image stabilization is working. Obviously, the switch is set to "IS". With my L series glass, there's usually a little click or whir and can see the glass move through the viewfinder. With this 24mm lens there is no sound or movement that I can discern. Is this normal for this lens? (Or the 28mm IS or 35mm IS for that matter?) How can I tell that if the IS is working? Is there a test I can do? Thanks for your input!