squeak when rear wheel spins backwards

A long shot, but push on the caliper to take the pressure off of the pads and see if it makes a difference. Perhaps just pads dragging weird????

The edge of the brake pad that points to the rear of the bike gets dulled by being the first point of contact that the spinning rotor, and any debris on it, makes with the brake pad. The front of the brake pad remains sharp due to just being the point where the rotor exits the pad. This results in a more curved surface of the pad in the rear, so when you spin the wheel forwards it makes a relatively smooth pass through the pads due to the transition allowed by the worn rear edges. When spun backwards against the leading edge of the pad, which is less worn, any bend of the rotor meets the pad relatively abruptly and makes a less smooth pass. If your wheel spins fairly freely forwards- a number of rotations before slowing down when spun by hand, I wouldn't worry too much about it unless you're doing long motos at a fast pace. The easiest way to fix would be to see at what point the rotor makes the noise (the point between the pads when the noise is heard), and try to bend it back more true while still on the bike so that the rotor stays centered for the entire rotation through the pads.