My SIG P6 does it occasionally. Now when I load it, I load it a hit the back of it against my hand. It forces the ammo to slide back against the read of the magazine, it resolves the issue.
Agree. I've had this issue with G19 mags. I don't understand what is rattling against what or why, but if I tap the back side of the mag against the heel of my hand several times like I am trying to seat the rounds flush against the back of the mag the sound goes away. Sometimes things just work and you don't know why.