When you are moving presumably, if so that sounds like a warped rotor or badly degraded pads. If it's a metal on metal sound then replace both rear rotors and pad sets. The very worst possibility would be a knackered wheel bearing, but you would notice the horrible rear camber and weird handling if that were the case. Either way inspection is simple, jack up that wheel safely (use wheel chocks since the P brake must be off) and spin the wheel by hand.