I've always wondered, but never found out which part exactly in a power steering pump assembly causes the pump to whine. I'm a Ford owner so they whine regardless..
I am still trying to figure out why some hydraulic pumps whine and others dont. The best I have come up with is fluid weight. I notice more whine in cold temps and have found some manufactures of pumps spec thinner fluid in the winter or have moved to multigrade synthetic fluids. Weather this translates to power steering fluid or not I dont know.