I very much doubt it's anythying to do with the handbrake - when pulling away, i.e. letting the clutch out and moving off from stationary, there's a clonk. It could be something in the drive train - a ball joint, maybe - something like that. I don't know what commonly goes on these vans. I've also knoticed something's not quite right cornering when there's a bumpy road, too - perhaps there's clonking then, too.