my cadillac seville overheated and starts but sounds like there's something loose. there's a metal clanking sound. could it be a piston? or a rod?
Without hearing the sound, it's hard to say. It could be main bearings, rod bearings, piston. The only way one knows for sure is to tear the engine down. If this noise was not present before overheating, the noise represents something internal and is serious damage. Engine ran hot for too long!