I have two 10 round mags for my P11. To my lasting dismay, one of these was plagued by the infamous 9mm magazine rattle, and one was not. The original mag that came with my P11 (in nickel for my stainless steel model) was the culprit. Beginning with the seventh round, the thing would start making noise like a baby toy.
I finally found another nickel mag that was new in the wrapper, but after purchasing it and opening the bag I realized it was dented (just a little bit) in two places. As it held all ten rounds and seemed to feed just fine, I ignored the imperfection, especially since my chances of finding another nickel mag were/are pretty slim. To my surprise, the dented mag did not rattle regardless of the number of rounds it contained. It then occurred to me that the dents must somehow be the difference.
Denting my magazines did not impress me as the best solution, so I started messing around, and found that upon loading the seventh round, the plastic follower was tilting, causing the rattle. Let me illustrate with some pics I took:
This pic show the approximate position of the rounds and follower after 6 rounds. This is the way it should load, and it does not rattle.
After 7-8 rounds, the follower begins tilting to the right, leaving a gap on the left side. This round is no longer under tension along its length, and will rattle when moved.
Here is the mag with all 10 rounds in place. While the positions in the pics are obviously exaggerated, you can see the rounds at the bottom of the mag are not tight fitting due to the incorrect angle of the follower, and thus will rattle.