Could this be a lack of oil to the rocker shaft? I saw this once a long time ago.
When you rev the engine it would push more oil and cause the squeek to stop. I don't remember if your 216 has a oil cap in the valve cover or not, but if it has one, look inside and see if you have an oil flow when running. Oil should run out the little hole on the rocker arm. You may have to pull the cover and look at them all.
