You have installed the Nock Out into your shaft, and you hear a slight rattle sound (like a loose field point) when you tap the arrow against your hand. This is because the silver activation collar is a metal injection molded MIM part made from stainless steel. The fit of the activation collar needs to be a little loose to allow you to quickly and easily rotate the collar from "Practice Mode" to "Ready to Fire Mode". You will notice that all of the collars will have the same feel because it is a MIM part.
If you are bothered by this noise, the solution is simple: Use a small amount of bow wax and push the wax into the grooves of the silver activation collar. Wipe off any excess with a rag and the rattle will be gone.