Scott,
Two possible - but nasty - explanations:
1) Broken piston - although usually accompanied by a smokey exhaust.
2) Dropped valve seat - bouncing around between the valve and the head.
Number two happened to me once - the noise had me stumped until the seat
jammed the valve partially open and the pushrod lost its connection with the
rocker - and surprise, surprise, the engine started to run really roughly.
Rick Harrison
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