the knock could be a worn hydraulic tappet or a worn cam lobe or both. it can also be a bad tappet(s) where the hydraulic bit is not doing it job.
to remove the tappets is not a big deal.
first remove the rocker cover and try and save the gasket and this will the only one you may need.
each of the 8 rockers will have a nut in the middle, simply loosen the nut and remove the rocker,but keep the assembly in order,watch out for a steel piece that either sits under the nut or under the rocker, it prevents the rocker from eating into the head.
with the rockers removed simply pull up the hydraulic tappet.
check for pitting and scoring, dishing on the surface that contacts the cam, if any then replace,
to inspect the cam properly then you need to remove it which is not to big a job but see how you get on with the tappets,but if any of the tappets is badly dished or heavely pitted you can be sure the cam has some damage aswell.
put every thing back in the same order of removal and each tappet in its original hole.