Dropped valve seat. The valve seat is a hard steel ring that is pressed into the head,when conditions are right..ie heat,age,quality of original fitment..the seat will fall out of the head when the valve opens and then when the valve closes it won't go back all the way. It won't come off the valve unless it breaks or the valve breaks, it just pounds the life out of the head everytime the valve is opened and closes. The valve spring pressure is probably between 200-300 psi which makes for a pretty good hammer. Type 4 engines and 140hp Corvair engines are known for dropped valve seats. That's my guess.