Hi, I bought used Sigma 24mm f/1.8 EX DG Macro for Nikon from eBay. I am worried because front element is little bit loose. So it moves sideways and makes a rattling sound when I shake it gently sideways. It works well with camera and takes ok photos in my books. There is little ding in lens hood threads so it looks like somebody has dropped it. What should I do? Have anybody this kind of rattling in their Sigma lenses? Thank you in advance!