Many, many guns do this. My P99 mags rattle if I load them to 15 rounds. STrangely enough - my SW99 mags (which work in the P99 and hold one extra round) do not rattle.
My PX4 that I had until recently rattled if ya loaded the mags all the way. This is just what happens for some doublestack mags. People claims that it can depend on how ya load it, etc. But, I find that no matter how I load these mags, they rattle if you fill them completely. That's just the way they are.
I have 1 less in my mag, as I always underload by 1 unless its a 10 round mag typically. So, I haven't had any issues.
My FNP-9 mags do this occasionally. One in particular rattles when fully loaded, but how bad seems to depend on the day. A couple of them don't rattle at all, but they are ones I just bought and have pretty stiff springs (which might be why they don't rattle).