loud rattling suddenly started while driving when in park it has a rough idle

rattling from where,inside engine,external from engine? have you opened the hood to try and find where the noise is coming from maybe a loose plug wire or a vacuum leak,broken engine mount?? distributor wires plugged in very good/tight..maybe a cracked distributor cap. PCV valve could have popped out,again hose loose,split,cracked,came off?