Tapping noise from the front of the cylinder head after head gasket repaird

This engine has manual adjusted valve clearances, have these been adjusted correctly?

I would try and isolate a cylinder that may be noisy, listen with a screw driver and see if you can pinpoint the spot that is noisy, one way is to disconnect the injector plug and kill off a cylinder, often this will alter noises and this can help locate the area of concern, as for what is actual causing the problem, it may be a broken valve spring, cracked or worn valve guide, I would double check valve clearances if you have not done so already, they may have been set initially but we must eliminate them as you can leave nothing to chance and best to be sure than go chase shadows.