Just noticed this yesterday. I've owned my M&P for just over 5 years. Since this is not my primary carry, I was just going to check for dust bunnies and lube. Soon as I removed it from the holster I noticed a rattle. I dropped the magazine to check a little more. Frame to slide fitment to me is off. Movement is side to side. The rattle is coming from the slide (end cap) and sear housing assembly on the frame. Any reason why the lockup isn't tight any longer?

Solved, back to S&W.