I had that issue in an 5.7 Mercruiser, 97 yr. Even after inspecting and adjusting the mechanical rockers, the noise would come back. It started to run rough. After rotating by the engine by hand and watching the push rods/rocker arms, lift was short on a couple of valves. Turns out a couple of cam lobs had worn.
I hope this is not your issue, it is an easy fix in DD and Easily replaced from Jegs.
If you have hydraulic lifters, you could have a bad one or two, that does happen. Varnish can build and cause them to stick and/or cause blockage in the oil passage, this can be rectified by pulling them, soaking them to clean them and then replace them, find a write up on proper reinstallation of hydraulic lifters. It could also just be mechanical failure. You can try to isolate the area by listening with a stethoscope, if you find that it is mechanical damage, you have the area of the bad ones, run to the auto parts store and buy a couple of new ones.
My rule is, if you have a problem start with least expensive part and work your way up, that's only if you don't know exactly what is wrong with it.