i recently also noticed that my mags rattle with 18 rounds.
After giving it some thought I came up with the theory that maybe, the magazine folllower, with 18 cartridges in the mag, does not have enough 'angle-play' to cope with the increasingly upward angle of the bullet row, due to the slightly conical shape of the parabellum.
It's just a wild guess, and I dont have enough experience with other brands of mags unfortunately to check if the cz follower is tighter or longer then others.
If this could be the cause, slightly adjusting the follower to give more play could be the sollution.
Maybe bending the bottom of the follower a bit inwards, or shortening the height?