The exhaust can also rattle a bit on the rear diffuser but this is not so much a metallic noise but a knocking. Easily adjusted by threaded mounting rods at rear. This case does sound like the heat shield which is easily fixed assuming you can get access under the car!!!
Had a metallic rattle from mine when new, only when cold; the exhaust was touching leading edge of front undertray, I sorted by just putting 2 washers above undertray.
