The 6" disk is 2900rpm and that is good. The belt is 435 meters per minute. so it would be about 400RPM (taking a guess) as the belt would be close to 0.9 meter
EDIT: i just took mine back and it was replaced no questions asked! The new one has no knocking noise! :D
@austy Let me know how the scroll saw goes. I was looking at getting one but someone suggested a band saw would be better. i don't have either yet so let me know :D