Its not uncommon for vehilces to do this, particularly commercials where less time is spent on reducing NVH (noise, vibration and harshness). My 115 has some resonance under load at higher rpm but not standing still.
Since you can get yours to do it standing still, we can rule out driveline components.
Next culprit will most likely be a piece of exhaust system coming into contact with the body somewhere. The exhaust should hand entirely from rubber mounts and the only place its rigid if onto the engine which itslef has rubber mounts to help damp out vibes. Suggest sliding under the van (well chocked etc) and getting one of Walsi's assistants to rev it up while you investigate (the van that is, not the other Walsi assistant!)
Also check for any heat deflection plates if fitted.
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