Yep. Sounds familiar. Mine has done this since I bought it. All brakes are in like-new condition. Park brake release (almost always) without issues. Just does it when backing up after it has set for a while. I don't think its rust as the bus stays inside where it's dry. Just cold does it. Once warmed up, no problem. My noise comes from either the tags or drive axle.
I know of others also that have the same issue....doesn't seem to a solution, nor does it seem to be a problem worth persuing...assuming that your brake system is otherwise in good condition.
I was told by a truck guy that the secondary shoe would try to grab when in reverse due to the way the friciton material wears. Sez not to worry, unless it starts doing it going forward when at operating temps.
Cheers, JR