There are several reports of heatshield rattles on various cars, sounds rather like that to me. They nearly always tend to be resonant, i.e. it will happen at certain frequencies which correspond to fairly well defined engine speeds. As the rpm increase there's nothing at first and then quite abruptly you get to resonance and the rattle gets very pronounced. It often then decreases again as rpm get higher and can disappear altogether.
There's a link to a video on here somewhere quite recently where the chap in a workshop shows exactly what it is. Someone will point you to it I'm sure.