Michael, yes it could be the propshaft but at least on my 4/4 the propshaft only touches the transmitting tunnel under hard acceleration in first and second gear and when loaded with two people and a full tank.The Plus4 offers some more torque and could be involved easier.
I have no knowledge about a standard Plus4 if she has anti tramp bars etc.
But what you describe sounds to me more like the leaf springs could make the sound and touch the frame on bumps when loaded with 160 Kg and a full petrol tank. This also was the case with my 4/4 and I changed for adjustable damper. It is not the most elegant way but it helps to keep the rear a bit more under control.
