Brake shoes develop a wear pattern in the forward braking a little rust and moisture just about all buses will chatter in reverse after setting for a couple of weeks, out of adjustment brakes the more the chatter, did not your Bendix instructor in Bendix class you were in not cover that ?
The Bendix instructor didn't cover much about wheel end stuff. I don't recall anything about chatter, but I don't remember everything from class either.
As to Chessie4905's point, this happens only when actually using the brakes. Dina uses a fairly standard Q brake system. Pulling any drums will have to wait until the weather warms up.