Just done a trawl through previous posts and can't find anything on this issue but it affected my previous Mk3 and this new one.
It's triggered when you go over a hump styled speed bump. It's not triggered by anything else and I mean that quite literally. It's not possible to reproduce it by bouncing the suspension and it doesn't occur if you drive over cattle grids or those much narrower speed bumps - the ones that appear to be made of rubber.
It's a creaky sound, it also has elements of a squeaky old sofa. My theory is that the bouncing movement engendered by these hump style speed bumps sets up a sort of resonance in the trim.
Does anybody else get this?