If you hear a metal on metal sound as you dry fire any gun, metal is striking metal.
Take a hammer, and a piece of steel, and replicate that sound. Even if it is a small sound, replicate it with your hammer and a piece of steel, 1,000 times.
Now in good light, look at your piece of steel.
Glock has changed their policy on repeated dry firing, and now warns against it without snap caps.