I don't have a Glock, but I believe they are double stack magazines, correct? It is possible with the one last round in there that there is enough free space around the rounds lower in the mag to allow them to move back and forth, hitting the sides of the mag. Probably doesn't affect anything, but it means that you get a rattle in your mags. I really can't think of anything you could do to stop this if it is a design issue of the mags themselves.
You could always just get a Hi Point and then it'll be normal for your gun and mags to rattle. :P