I have the same rattle on my 77 and the tech's reckon it is the fuel lines rattling in the chassis frame, and how they fix this is by removing said lines and wrapping them with a spongy type tape in sections and re fitting them, alternatively they said [for a quick fix] i can shove a rag down there to prevent the pipes from rattling...not liking this idea much, as i like to have repairs done properly. Car is getting looked at next week