One problem related to brake grinding noise has been reported for the 2012 Toyota Prius. The most recently reported issues are listed below.
Repeating several-second groaning or grinding noise heard from rear wheel area upon startup motion. The noise is severe and sounds like low-pitch metal grinding. It's potentially brake related but could be suspension bushings or other components rubbing against one another. It does not sound like typical brake glazing. The noise occurs in all weather conditions and typically after vehicle has been sitting for several hours or overnight. Noise continues for several minutes at stop and start. Noise is loud enough to startle an experienced driver. The absolute severity and indication of the sound is enough to question safe ongoing functionality of braking and/or suspension systems. Multiple reports are documented online at Priuschat. Com. Toyota does not appear to have a fix or resolution at time of writing, 10/2012. This is for 2012 Prius v model five.