day whip

I have a 2005 subaru legacy 2.5i automatic sedan. i was driving it today and pulled it into the driveway stepped out for a second and stepped back in. When i put the car in reverse gear it emitted a squeaking noise like a slipping belt. i immediately put it in park, and the noise went away. once again i put it into reverse and the noise resumed. all of this was with my foot on the break at a stand still. any ideas what it could be? Here's what it sounds like: