This sounds like what my m8s car did its sounded like tappets so he got a new kent cam and lifters......(as you do )
it wernt that then put new rings pistons in to remove piston slap (as you do again )
surprise suprise it wernt that, then put new bigend shells in it............... it wernt that either so he swapped the lump and gave it to me for nowt.
i removed the only thing he didnt swap?????
the mains had become worn due to poor oil pressure from a blocked oil strainer/pick up pipe so 25 quid set of mains and it was back on the road and quiter than ever (bargain )