CVH top ends are poorly designed and slowly wear from the moment the engine first runs, the reason they start to tap is because the cam lobes wear, and after a while start to dish the bottoms of the lifters. As a result it gets to a stage where the lifters can no longer soak up the clearance in the valve train, and hence the tapping.

Fitting new lifters alone will last all of a few minutes until the worn cam has munched them up again