What I found may help others:-
I have just traced it to a worn rubber grommet in the bracket that holds the turbo air pipe to the chassis on it's journey from the intercooler at the front of the vehicle. It is situated roughly level with the front of the engine and can only be accessed from underneath with the tray off. I found a suitable repacement grommet and wow what a difference!!
Because the grommet had split, the bolt holding the pipe was making metal to metal contact with the chassis although it looked fine from the outside.
Result? When turning off now, no banging or knock - when ticking over, no rattle and buzz - when driving, much quieter all round!
It's amazing how such a simple job can have such a major effect.
Also while I was under the vehicle I tightened up the bottom bracket holding the central fan assembly by simply bending it up a bit with a few turns uf insulating tape around the mounting peg - again a simple cure for annoying rattles!!
Happy bunny now!
Malc.