I noticed when I loaded my .40 caliber mags with 14 rounds today they had a "loose bullet" rattle sound.With 13 or less in them they were fine,no rattle.They did not do this before I sent them back with my pistol to S&W.Does anyone else have this problem? BTW,my 9mm mags don't rattle like this.

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How many guns are too many?

This is common thing with just about all double column mags. Not that all of them rattle -- but plenty do. It is simply a function of the fact the short, tapered cartridges in side-by-side stacks are tricky to box up, and if a mag is slightly large at some point, it will rattle. I have five mags for my 6906 -- two rattle, two don't, and the fifth -- sometimes no, sometimes yes.

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Pisgah

I'm just wondering if S&W shipped my pistol back with new mags??????? In my post earlier today,about getting my pistol back,I stated how much easier and smoother the mags went in.And the rattle sound at the range today,they were not like this before I sent them back.Hmmmmm...........

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How many guns are too many?

I have four 14 round mags for my Sigma and two of them will rattle fully loaded while the other two won't. Funny thing is...one of the "rattlers" will only rattle when loaded with my "Ranger" carry rounds and not the FMJ target loads. It must be the gremlins at work...that or it's some kind of Communist plot. Or maybe........it's both.....hhmmmmmm...

LOL! Maybe it's a conspiracy theory.......hmmmm...

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How many guns are too many?

Very common with any double stack mag. I think it is the random way the rounds will feed into each row of the mag from the top.
Just smack the mag base floorplate into the palm of your hand, should settle the rounds.
I think you may find they dont rattle much when loaded into the pistol.

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2nd Admen.-Right to bear AR 's

Your double stack magazines did this before, you just didn't notice. It's not an issue, don't worry about it.