How would changing the oil pump do anything for your lifters again? Remember oil pumps don't decide oil pressure, it's your engine's tolerances that do. The noise went away when you topped off the oil? Or when you added the MM oil? I've seen atf and/or MM oil clear up lifter problems. How long have you had the car? What's the oil change history like on it? You might be better off doing a set of lifters.