Originally Posted by FCross7
What you're heading is probably the striker moving around. If you cock the pistol does it do it?
When the trigger is in the rearward position, meaning the weapon isnt cocked, the trigger bar depresses the striker safety allowing the striker to move back and forth. It's totally normal.
+1 Fred is correct... But...
Could also be the rounds in the magazine moving, slap the bottom of the mag and reinsert it to check if there's the "rattle" as it's commonly referred to... The rounds don't all sit flush and perpendicular to the bottom mag plate due to the angle of the mag, the rounds towards the bottom of the mag sit facing slightly downward and the rounds at the top sit facing slightly upward so they can hit the feed ramp to be put into battery... The result leaves 1 or 2 rounds in the middle of the mag loose fitting and can cause rattle, smacking the bottom of the mag will make it go away
Really a non issue though as both the firing pin freely moving and the mag rattle doesn't hinder performance at all when it makes noise