This is one of those odities you come across. My Sig Pro does the same thing. When a mag is inserted into the gun, slight pressure is put on the top round in the mag thus changing the pressure inside the mag and causes a round to rattle a little bit. If you remove the mag and press on the top round a little and shake, you can usualy recreate the rattle. Only one mag on my Sig Pro does it, the rest are fine. Maybe a spring change/adjustment may do the trick. MY M&P does not do this.