kbr80: It turns out I'm having the same problem you are.
As I mentioned earlier, I've been looking for 16 round magazines for a couple of months, after I ordered one from WaltherAmerica. None of the gunshops in my area had any.
I was about to try ordering online... Earl's Repair Service sells them for $75 and CDNN said they have them for $40.
I was about to buy a couple from CDNN when by luck, I dropped by a little gun shop I never go to (not very good selection and not very competitive prices).
Amazingly, this place had 5 of the 16 round magazines, at a competitive price ($40) and I picked up 2. They came in Walther packaging (no Frankenmags) and had their UPC label/seal unbroken. I brought them home, cleaned them up, and loaded them up for a preliminary function check.
As I was loading them up, every few rounds I would hear a little rattle. A few more rounds, and the rattle was gone. A few more, then a rattle. When fully loaded, the magazines didn't rattle.
I remember thinking, "Damn! This sounds just like what kbr80 was talking about."
I'm not sure if the springs need to be cycled a few times, but I loaded and unloaded them a few times, it didn't seem to do much. Right now, they are loaded up full and I'll leave them that way until I can head to the range to see if they function properly.
If they function well, I might ignore the rattle. The magazine bodies and floorplates seem fine and that is all I'm really looking for. The reason is I plan to order a batch of magazine springs and followers from S&W/WaltherAmerica to keep these magazines working for a long time.
If you are interested, their part numbers are:
* 16 round P99 magazine spring - Part 2628015 @ $4.47 ea.)
* 16 round P99 magazine follower - Part 2618265 @ $4.47 ea.)
I called WaltherAmerica to tell them about the difference in magazines (the one they sold me vs. the one I bought retail) and to find out if I possibly got a batch with some bad springs. They mentioned if I had problems after I tested them on the range, that they would be glad to look at them and get them up and running.