Could be lots of things. Does it get better/worse when the engine is warm/cold or when under load. Where abouts does the rattle come from in the engine bay? Could be somethng simple like a broken exhaust mount, or somethnig worse. I've got my fingers crossed for you!
to me the rattling is hard to hear where it is coming from because it sounds like its coming from all over. how do i do a full oil change and put a new filter on because i will do this 2morrow. i will also bleed the coolant system out completley and refill it all correctly. please could some one tell me how to do these