Normal? Yes. What is it? Haven't a clue. My guess is the rounds bouncing around until they rotate up to the top of the mag to the feed position. I've got a bunch of the 10 round mags and they all rattle when loaded.
Well answered RNGreenHat. The Ruger rotary magazine is noted for round slap.
Worth adding as an aside that the shells in most double stack center fire magazines (unlike single stacks) don't align perfectly as loaded and will often rattle somewhat - cured by hitting the mag base and then the mag back - otherwise known as the Aussie slap