If you take a look at the Millenium Pro posts or smithing this was just discussed this week, in the PT's. Apparently it is not an uncommon occurance. Try smacking the full mag against the palm of your hand to see if it helps seat the shells. Also if it only does it when full, and not when the top round is out.
it's not a problem. just seems to be a quirk of the double-stack mag. when it's fully loaded, the rounds line up in such a way that they're all forced against each other except for one round. this round doesn't have the other rounds touching it with any pressure, so it wiggles back and forth. on my pt140, it seems to be round 8 or 9.
it might actually be round 10, as the follower is forced against round 9 and isn't putting pressure on round 10. round 8 is pressing against round 9, so it doesn't press against round 10. if you take one round out, the geometry changes and all rounds are touching each other.
My Keltec does the same when it has 8 to 10 rounds in it. It holds ten. I've heard of guys that just take a pliers and GENTLY squeeze the mag at the bottom. Just a little at a time. I only load mine to 8 then chamber one so no noise.