Actually heard this in another place now!
Where I'm working currently is a large estate and a long drive to the car park along rough and bumpy roads!
One particularly rough section, but not bumpy, it did it!
So its a combination of slow speed, no more than say 20mph and road surface.
Much as I hate to say it but Wayne is partly right!

Must be suspension related. But dont believe it to be strut mounts, maybe ARB mounts, because of the way it rattles.
So will have to get under again!