I had a problem at RC once where something had lodged somewhere between the front rotor and probably the backing plate. That made a loud squeal until it fell out (before I had a chance to find it and locate it exactly). The squeal was noticeable while the jeep was moving, with or without brake.
Although, I would have thought something like that would have cleared itself on the ride home at least. Anyway worth a look to see if something is wedged in there. It's probably not that easy though.