One way to start the diagnostic process is to find a level, straight road with a good surface and very light traffic. Take the van up to about 60 mph, shift to neutral and shut down the engine. Be mindful that the steering will be difficult with no power assist but that should not be an issue on a straight road. As the vehicle slows note the speed when the noise is loudest. If the noise disappears when the engine is not running but vehicle is coasting, then you know it is not a driveshaft or differential or transmission noise but an engine noise.