I have had this problem before: When the engine warms up, the muff expands and if the head joint is not made correctly, the liner moves up and down with the piston and the resulting noise is as you describe.
For a total cure you will require a modern, well made muff complete with a liner with the correct interference fit.
The liner top flnge thickness must be compatable with head recess so that when ground together they have total contact.There should be a gap between the face of the muff and head when the grinding is complete.
Hope this helps,
Robin.