Could be imbalance or detonation, could be a miss... could be bad valve springs or could just be the tune of the carburetor but, I'm guessing you'd be hearing it too
"I pulled the transmission and the previous owner installed a neutral balanced torque converter on the stock flex plate. I purchased a B&M flex plate designed for the external balanced 360LA and a new Hughes neutral torque converter. The vibration is gone! Thanks for the help."
