My z440 fan cooled has started knocking after riding it today. This is my first time posting for help. [https://www.youtube.com/watch?v=sQxxMqZqc_s9] I made a video for it its really bad at lower rpms. I'm not sure if its the crank. Pistons looked really good for 6600 miles on it. Any help would be appreciated on what it could be.