Mine had the same issue, if you mean a rattle/growl at low revs which disappears at higher revs.
The problem was with the backlash eliminator spring on the output shaft. I'm not sure that's the proper name for it, but its function is to ensure quiet engagement b. the gears. The part is cheap and it's a snap to install. It's just getting to the cursed thing, followed by the ordeal of reassembly. Replace the rubber dogs while you're in there, too.
If this is indeed the problem w/your bike and you take this route you'll be floored by the quietness/smoothness that results.
