When the rear axle goes into compression - especially if the van is loaded up - we sometimes get a real heavy 'bang/thump' from somewhere under the middle/rear - sounds really terrible when it happens, its not a light knock - its a proper 'bang'.

Trouble is I can't trace it to save my life - its not something you can make happen on the driveway because it needs the van to really drop into its spring travel to do it - so it always happens when we are barrelling along.
Anybody got any ideas of what might be the cause and how I might/could 'test' for it?
Any suggestions gratefully received!