Aardvark its not the back box bit. In my case it was just after where the mid section joins the rear section (pre back box), ie about half way along the car slightly to the left. The sound transmitted such that it appeared to come from the rear right for some reason as well as underneath. I can send pictures if you need and sure Iain knows where. The undertray front is held by a few bolts. If you look from the front the right hand side is where the exhaust runs just next to the tray. The supporting arm of the exhaust runs within about 5mm of the front edge of the first fin of the tray and as the tray flexes it clatters the arm and transmits a rattle throughout the tray. For me simply putting two to three washers in between the tray and car on the nearest bolt to the exhaust pipe moved the touch point far enough away such that a wheel hop whilst booting it over bumps doesn't make any noise at all.
Might be the same for you..if MY 10 and Milltek