as noted this is most likely the sign of a loose screw or something. Most often this can be found in the sight. I usually find the rattles by banging the heel of my hand on the accessories and see if one buzzes or rattles when I hit it. A bent axle can do this as well and is harder to diagnose if you are not real bow savvy. One last thing to consider is a Severely stretched cable allowing a limb or wheel to have some slack/vibration upon release.

Just Some Starters.