Low pitched squeal when backing up

When I first back my car out of the carport I hear a low pitched squeal when I hit the brake. It sounds like it is coming from the rear. When I drive a block and hit the brake to turn on to the main road, it does not do it. Once the car has warmed up some, it seems to stop. If I stop at a store and pull into a parking place, then come back to the car and back out of the space it does not make the noise. It only seems to do it when the car is cold. The brakes feel like they are working fine.