On my viper I had the same problem...I looked and the you have the regular chin guard that you would find on a bike, but I aslo noticed a guard on the bottom like a skid plate for the disc brake. The slack in the chain was hitting that. I adjusted the chain to have the recommended slack and problem fixed.

 

Sorry I meant to say that my son's viper has the regular chain guard that you would find on any bike, but...