Ran into this exact thing recently on a friends 228 GMC. It turned out to be the front damper was loose on the crankshaft. It clattered on acceleration and deceleration at certian rpm's. Don't pull the pan yet. We also thought it might be wrist pin or #2 piston. We changed out the damper and put in a new key ~ problem disappeared. Good luck.
"Drugs are money, money is power, power is security." - Kent Heiner
Nedrow population 2212
Carbon footprint family of 2/11 tonnes of CO2/yr
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