Knocks caused by excessive clearance such as rod bearings just don't go away. I don't know what you heard but I would not worry about it if it is gone. If it happens to come back, try to pinpoint the sound by holding your ear against the handle of a long screwdriver and put the other end on the valve covers, manifolds, etc. until you pinpoint it. Then you might can determin what's causing it. A fast ticking at idle sounds like something in the valve train to me. It also could be caused by an exhaust leak, but they usually will not go away on their own either. By the way, welcome to AFM!!!