Roger,
I would start with doing a thorough tuneup, especially the valve
settings. After setting the valves, wiggle each up and down to
see if thay all seem to have about the same clearance. Sometimes
rockers lose their hard surface and the valve stems dig holes in
the noses. Under these conditions, it is very difficult to get
the lash set correctly.
If the noise only appears at a certain RPM regardless of the
gear you're in, you might be getting a hormonic vibration. So
check everything for looseness: starter, engine mounts, steady
brace, anything attached to the engine or in the engine bay.
However, this problem may be hard for us to diagnose without more
information such as:
Loss of power.
Greater or a change in engine vibration
Blake
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