Since it's more of a home defense gun than anything and sits in a drawer I've just ignored the rattle but I'm curious if it's fixable now.

When the factory magazines are fully loaded there's a loud rattle whenever you move them.  It's especially loud in mags that are not in the gun.  It literally sounds like there is small rock bouncing around in them.  If I unload them the rattle goes away.  If I take a couple rounds out the rattle isn't nearly as loud.  I assume that others have dealt with it since all my mags for this gun do the same thing but I don't know for sure.  

Are my M&P mags possessed or is this a known issue? Can it be fixed so I can have full mags that don't rattle?

Have several M&P .40s & 9mm.

Never heard a peep or sound of any sort

Have you removed the magazine floor plate & cleared - cleaned out the magazine?

Mine is loud if loaded up, if I only load 13 or 14, no rattle. It's kinda weird.

Mags are completely clean and cleared.

Yup,  it is definitely weird.

I have a couple Glock gen 3/4 mags that have a rattle when loaded.  I took them apart, everything looks good.  Never had a functional issue with them.  I assume when the spring is under tension part of the spring it is poking out and bumping the inside of the mag.

Yeah I have noticed it.
As you say in the gun or in a mag carrier with pressure on the top round it pretty much goes away. I don't worry about since that is the case.

I have about 30 mags for the M&P 40 and only some of them rattle. If it bothers you try different mags.

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.

Mine do it sometimes