The valve clatter would be less evident as the rpms increased.

One other area that sometimes makes the clatter noise at idle is the starter bendix.

 

I agree with 425Xpedition. Mine has had this noise since a bought it 3 yrs ago. It sounds like a marble rolling around at idle ( that was the dealerships mechanics words). He couldn't explain what it was. I've since had the recall done on the clutch, and everything was in good shape. It is definately coming from the clutch side of the motor. Just another one of the mysterious polaris engine noises.
Brian