this is what happened exactly in my 72 charger. it shook like i mofo between 46-58mph. then one day i was starting it up to go back home and pop bang bang rattle rattle rattle....nothing. i blew up the starter gear due to the flex plate cracking all the way around. that was a good day...not! that car is long gone, but still if anyone has these issues and its NOT what was listed above (mounts and what not) its probably interal. be safe and get it checked!