I've only had limited experience with these engines, but I wouldn't get into any engine work unless you have a lot of experience, and know about the construction of the engines. They have removable cylinder liners. If you have to remove the head to get the lifters out, you will probably have to remove the loners to replace the seals at the bottom of them. This is a total engine rebuild.
A long shot for the noise might be the hydraulic pump. These Rolls-Royces have a hydraulic-pneumatic system, licensed from Citroen. French engineering, British craftsmanship. What could go wrong?