OMG Iv'e been having the same exact problem with mine for a while now! I tried taking the back off and tighten everything the best I could but it still does it. Mine can be tight to focus at times, and there's some "slop" "wiggle" to it. When I hold the lens in my hand and move it around it makes a clunking sound like the barrel is loose. There's a shop in Providence RI that I'm thinking about contacting. It's only 1 1/2 hr's from me so I may see if I can bring it down and wait. Spend the day down there instead of mailing it.
Can you tell if it's effecting the sharpness of your images?
Not right now. In a couple of days I can get my hand on a Nikon 5100 so I'll do some pixel peeping. If it is ill it might be the CRC group. That should affect near-distance performance, right?
I photograph things to see what things look like photographed.
- Garry Winogrand