The steering hydra-motors are usually quiet on those, especially when in neutral. I'm not sure what could be doing it, possibly there's too much pressure coming from the flow divider. When the steering is in neutral, oil still flows through the hydra-motor, and on to the oil cooler up front, and then back to the reservoir. Could be a kinked line, or a hydraulic hose with a 'bubble' in the rubber that is blocking the flow.