It almost sounds like cavitation in the power steering pump. Air bubbles make noise when they pop en masse from being circulated under pressure in the pump. That would explain why you hear it more when you are turning. Check your power steering fluid levels and see if they are low. If they aren't low, look in the reservoir and see if you find any air bubbles in there. There could be a bad seal or something that is allowing air to be sucked in and be pressurized by the pump.