Stop the bus , very possibly a broken or cracked flex plate , I cant say i ever saw a neon with loose bolts on the converter . you should be able to pull the inspection cover between the oil pan and bell housing , often you can see cracks and rust right near the crank hub . if it was a rod the dishes would allready be done , i have seen pistons on those make noise , by pulling injector wires you could isolate it to a specific cylinder .