Mine is Cast iron 1993 model Enfield Standard, I have one query, What I mean to say is even iI have the same Tappet sound, but if we are not tigetingtightening the tappet fully it will effectaffect the engine, so if the sound is coming means it doesntdoesn't mean like your engine have any issue, as I knowsknow, you should service from a good mechanics, and ask him how to effect this for engine rather than reducing sound
Regards, Jay