Some time ago, one of my friends also had a noise from the rear wheel area, and it was the brake drum loose. Unlike conventional wheels which hold the drum on by virtue of the wheel nuts, with wires of course, there is no contact. So we rely on the 4 set screws and star washers. In his case, they were indeed loose, so always worth checking whenever the wheels are removed. When replacing, I always apply a drip of medium hold locktite to the threads to be on the safe side.
