Yes the sprocket is designed to be loose on the countershaft, though I don't know why. Seems like it would wear out the splines a lot quicker. I liked the ones with a large bolt holding them on. They didn't move, but were harder to get off. As said, check the chain adjustment, if that is ok, check the teeth on both sprockets, and see how much adjustment room is left on the chain. If there is almost none, the chain is worn out. If the chain and sprockets check out ok, and the adjustment was correct or you adjusted it, there may be an internal problem. But a "clunk" usually comes from quickly taking up slack in something.