Since I had the MulFab sump fitted the only time the car grounds is if the gearbox cross member hits: it now has a slightly abraded lower edge...
As Jay says, there isn't much else exposed and it is hard to think what might be causing the symptoms described. My only thought, and it is a wild card play, is that the gear box cross member has cracked on the side where it joins the chassis rail, allowing the gear box to move when turning and so setting up vibration in the prop shaft. But I cannot believe that one bang would do that, there must have been pre-existing damage.
