Quite to the contrary with my M&P mags. When they're full, there isn't ANY room for movement in both 9mm and .45. Even -1 or -2 rounds, no rattle what so ever. Now, my 7 round officer size mags on the other hand, they all rattle like ball bearings in a coffee can.
From what I've read some rattle and some do not. I know a few rattle until you put them fully loaded into the gun so who knows? So far, none of the ones I have rattle.
i have all 3 poly frames.....M&P 9 does not rattle,,sigma 9 no rattle, SD rattles like bad doors on a 67 Buick
called S&W about it and they said that they had a few other comp. make the mags and some were just a hair bigger but will work A-OK in the gun......i took a mag apart and stuck 1 small piece of black on the inside back wall....no more rattle..no problems with feeding either,
good luck
I have a theory about the rattle. I believe it is caused by the rounds of ammo in the magazine not seating properly against each other allowing some wiggle room if you will. I have a 9mm Kel-Tec P-11 with magazines that sometimes rattle. I find that if I give the side of the full magazine a good rap or two against the palm of my hand the rounds seat themselves and no more rattle. By the way, my M&P 40 magazines do not rattle... or have not with the ammo I have used so far.