Sounds like a Valve Train Issue , since there is a noticable power drop upon warm up . I would check your tappets before you punch a piston dome with a valve .
Note: Take a large flat head screw driver and gentley place it upon the Valve Cover areas and listen for where the noise is coming from by placing your ear upon the handle of the screwdriver. You can also check for wristpin or bearing issues in this manner , should the valve lash be within spec's.