I have a couple of Walthers and they both rattle a little, the 9mm rattling less than the .40. They did loosen up slightly from when they were brand new. They loosened up after the very first 200 rounds through each, but they haven't gotten any looser. Break-in, you know. Yes, I'm a lube freak, so they were not fired dry.
All guns loosen up with firing. Even those tightly fitted custom 1911s. You can even take an out of the box Les Baer (generally recognized as one of the tightest guns out there), put a few hundred rounds through it, and it will be noticeably (albeit slight) less tight than it was before you shot it. Don't believe people that say they have a (insert brand) pistol that has 10,000+ rounds through it and is as tight as the day they bought it.