I'm sorry to have to say this but odd squeaks and rattles are fairly common place in these vehicles. Once you've tracked down one and cured it, another will pop up. Some are easy to diagnose and fix whilst others are almost impossible to locate.
Often they are caused by the contents, so if all else fails empty the van and see if it persists. Whenever I've had these problems in the past I've asked our eldest son to travel in the back and listen for the noise. He is mechanically minded and so far has found the issues that annoyed me, e.g a rattling rear window blind.
I repeatedly reported one rattle to our dealer but after several attempts and some new parts the result was no different. They also gave me the impression that their hearts weren't in the job! There's only so much time they are prepared to spend on dificult to diagnose, Warranty items like this.
Try to record the details of speed, road type, location and conditions when the rattle is at its worse, e.g low speed, single track road with poor surface. Then you should be able to recreate the same conditions for when you have an assistant. Those little sticky backed felt pads are very useful for stopping rattles but also eliminating potential sources.
Common sources are:
Window blinds
Awning
Sliding door
Good luck.