So I did some more research and found that this is a very common complaint with M&P40's.
I have semi auto handguns from several different manufacturers and yes, some mags have a slight rattle. None of them come anywhere near the M&P40 rattle though. It sounds like there's a rock bouncing around in the mag. From what I've recently read, nobody has figured out a fix for it so it is what it is at this point. The gun is staying in the drawer so no big deal overall. It's like having a toaster that barks. It doesn't affect the toast but it's definitely annoying.