My motor ate a cam follower a couple of years ago. That sounded like a valve seat gone bad but not as loud, I've had that too. Not quite as bad as the article in the tech section but close, out the bottom instead.
http://www.pelicanparts.com/techarticles/914_valve_blown/914_valve_blown.htm
Check to see if just one of your valves is way out of adjustment. Then make sure it isn't the adjusting screw and that you do have compression.
If you have compression chances are it isn't the valve seat. If the screw is tight and in good condition its probably not that either.
I've heard of rockers breaking but I doubt its that or you would definitly know by now.
The only way to know is to pull the follower out. That can be done with the motor in the car. Just take off the rockers pull out the push rods and tubes. Then stick you finger in the hole and pull the follower out with your finger tip. If you put a rag on your finger it helps grip. If you cant get the follower from the affected cylinder out, I would say its time to open the case(engine case,that is).
I would recommend cleaning the whole area first before you start taking stuff apart though.
Conrad Peden
72 L20E in rusto.
73 L20E in resto.
[This message has been edited by Conrad W Peden (edited 11-19-2000).]