I've had nightmares with SA hubs and rattling, but I think that the newer types should be less prone to it than the old AWs.
I'm guessing that if you hold the bike steady and try to rock the rear wheel you'll detect a small amount of play in the bearings (you have to make sure any movement detected is coming from the wheel and not the rear frame hinge). As grease migrates from certain areas and the bearing races wear fractionally the internals are given enough space to move slightly, and riding over rough ground results in rattling. Readjusting the left hand (non-drive) cone so as to dial out the play ought to get rid of it.