Thanks. I disassembled the mag, inspected it carefully, and reloaded it. 13 rounds, no rattle, 14 rounds, no rattle, 15 rounds, no rattle. Ahha I thought. Unloaded it completely and reloaded it - rattled on 14!!! Oh well. Isn't gonna effect things functioning so I'll ignore it. Thanks for the response again.
BTW I love your products. I'll be ordering a 6" barrel for the 20 as soon as I recover from the expense of the the Glock.