I've heard this on a FCR before, turns out it had the lightened throttle spring. Take your air boot off and watch the slide when it's idling, it's probably chattering up and down. We had to change back to a stock throttle spring to get it to stop.
+1Carb flat slide chattering is typical from what Ive heard. You just dont notice it until you remove the airbox or lid. Its not anything to worry about.[/b]