Clickety-clack sound when going over bumps

[V12noise]

I have this sound from under the car when going over bumps. I can only describe it as a clickety-clack as if one of the undershields was loose and flapping. It isn't that, as I have checked all the plastic and thermal shields and even wedged bits in there to be 110% sure.

With the windows down, it appears to be coming from the fuel-pump area, i.e. just in front of the left-rear wheel. With windows up, I can't hear it.

Neither the annual technical control nor an independent service mechanic could trace it. The previous owner fobbed me off by saying it was the rear left brake clip that was loose - I didn't believe him but thought I'd track it down soon enough. Unfortunately I haven't and it's driving me nuts.... any ideas from anyone?

2010 X5 35D