I am pretty certain something is loose on a rear wheel. I get this dull rattle sound if the van moves over speed bumps etc. is it likely to be a swing arm ?
citroen berlingo longer wheel base. 2015. I did have a new exhaust fitted last summer and under carriage as a rock scrapped the whole bottom of the van.
Can you look at the back of the van, and see if the rear wheels are showing any negative camber, i.e in at the top, sloping outwards at the bottom?
Does the noise sound more like a boing ( spring noise) when you go over a bump?
John