Low batteries usually are the cause of starter solenoids "rattling." The low battery can't deliver enough power to set the solenoid. Note that Harleys have an "armature" in the primary case that acts as both generator and starter.
Most Japanese bikes have a solenoid-clutch mechanism and a separate alternator, which produces enough power to run through a selenium rectifier and voltage regulator -- usually in one compact unit. When they blow, the only cure is to replace them with new parts -- avoiding rebuilt like the plague.
