Yamaha blaster knocking noise please help?

hello my Yamaha blaster slightly knock when in a high gear with low rpm but when in a high gear with high rpm never knocks or it doesn't knock when idling its a 2002 model was recently fully rebuilt some of my friends said ignore it it was just rebuilt but a knocking cant be normal any help would be appreciated oh by the way it doesn't happen when im accelerating fast but it does knock when im accelerating slowly it got a new crank and bearings and piston and rings about 15 hours on them