No smoke at idle, just a lot of racket.I still can't see any scenario where the crankshaft would break in a way where it would be noisy at idle and quiet at speed. I have seen out of round rods that were quiet at idle and got progressively louder at speed. Can't imagine a break where the bearing play would be increased. You could spin a bearing or destroy a bearing, but that isn't going to get quiet at speed.
An injector that is overfueling can produce a real racket at idle and might clear up when the engine speed got to the point where it could handle that fuel. Are you getting any smoke at idle?