I recently purchased a Mamiya Sekor C 50mm f/4.5 lens (for a Mamiya RB67) from eBay. I received the lens last week, and the glass is clean and the body has a few scratches as described. The lens seems to function as expected, but when I don't have the lens on the camera, if I rotate it--either end to end or turning it like I would if I were to screw it onto a camera--it makes a clunking or rattling sound. Can anyone else who owns this lens tell me if this is expected for this lens type? Or did I end up with a dud lens that I should attempt to return?