I have three magazines and all of them have a very irritating rattle when loaded with 8 to 10 rounds. The rattling occurs near the follower. I had to fix this one.

The magazine sides need to be compressed together in the area of the rattle to eliminate this rattling. I have found that a depression of about .005" on both sides of the magazine will do it. There are probably many ways to perform this operation in a controlled manner. I would like to hear some suggestions.
I used a Panavise which is a small vice that does not have much leverage. There is a 1/2" wide plastic strip as the jaws. Perhaps less than 1/2" inch is more optimal to produce the desired depression. If you use a vice, make sure the width of the jaws are no more than 1/2 width of the magazine. I positioned the jaws parallel with magazine, centered against the sides with the small holes. The main point of contact of the jaws is from the 5 hole to just past the 10 hole. The magazine narrows towards the feed lips and this portion is never touched by the vice jaws. The vice jaws never contacts any edge on the magazine. If I'm unclear here, then please don't do this mod.

I then squeezed that area of the magazine together. To be safe, I constantly checked my progress by loading the magazine and checking the depression with a straight edge perpendicular to the magazine and checking the gap with feeler gauges. The rattling decreased then ceased when the depression reached about .005" on each side.

For this operation, it helps to have a magazine loader. I used a "modified" HKS 941 loader. I put a groove into the "handle" of the HKS loader so that a medium sized tie wrap could be installed. The tie wrap is at the right point where the base of the shorter Mecgar magazine needs to be supported.

Bill Daggett wrote this KTOG posting with regard to magazine rattles.

Some of the Mecgar mags (although high quality) are too wide which allows the "double stack" of 10 rounds to have less height than normal. Take a pair of Channel-Lock pliers and a shop rag and place the rag around the empty mag. Then GRADUALLY squeeze the mag with the tips of the jaws on the pliers all along the front and rear sides 3/8 inches in from the edges of the mag. You may not see any deflection at first. Go slowly. As you see a slight deflection, reload to full capacity and shake vigorously to check for rattles. The front edge seems to make the most difference because of the taper of 9mm rounds. (The rattle is from the looseness of the bullet end of the stack) Mine was really noisey and you could hear my P-11 rattle when I walked briskly or jogged. Now it is totally silent.

BTW, if your mag DOESN'T rattle but you are jamming occasinally, it might be too narrow causing tightness and slow feeding. Just a thought FWIW.