What causes squealing from rear wheels in reverse?

Should be moan, not mont

Not uncommon. The brake pads are "used" to working in the forward direction almost all the time. Try using some anti squeal compound on the pads.

I have a manual tranny so the brakes are not applied when backing up. Also it is doing when slowly going forward as well. Still think it’s the brakes? Could it be something more serious like a cv joint or u joint?

Hard to say until you get the car in the air and inspect it. Maybe your emergency brakes are activated? Check the cables and associated parts.