ok, just picked up the new M&P 40 Fullsize.

I filled up both mags with 15 rounds.. but around the 11th, something seems to rattle in there.

funny thing, when i fill it up, sometimes it does not rattle, but when i put it in the pistol it will rattle again, and then sometimes it just stops..

i tested both mags, almost the same excat senerio.

ideas?? i cycled all 15 rounds and it looks good..

BTW when I put it in the pistol, it's 15+1 in the pipe.

e..b

Can't speak for the M & P specifically but rounds rattling in mags is not uncommon.  Harmless.