Well follow the steps to figure out what is causing it. first pull a plug wire at a time if it makes no difference in how the engine is idling/the noise then put it back on and move to the next.. when oyu locate the cyl that is missing pull the plug and look for mechanical damage.. then pull the valvecover and check the mechanicals (rockers too tight, pushrod bent etc.. )

Something is either causing the plug to fire when the intake valve is open, or the valve to not close completly..