Engine/Trans Seriously Bad Noise

Could the starter drive gear be making contact with the ring gear? I had that happen once. Sounded like a machine gun. The starter had one abnormally large hole (been so long I can't remember if it was the upper or lower hole) and I loosened the bolts and rotated it away from the engine and the noise went away.

BTW: a flex plate can't be installed backwards. The crank bolt holes won't line up.