Could be a number of things
1.have you checked to see if the spark plugs are all tight?
if not check they are all snug as they have known to work they're way loose and cause a tapping noise
2.could be a noisy injector
3.could be that its some head work in the past and been poorly repaired.could be the result of a headgasket going or snapping a timing belt.or the crankshaft pulley could have slipped and messed the timing up
If it number 3,Welcome to tapper-ville,population YOU!!