Why does power steering whine?

There are three reasons why you would hear a whine in power steering. The first reason is when the steering wheel is turned to far. The fluid flow to the pump is cut off forcing the pump into pressure relief. The pump has to recirculate the high pressure fluid within itself because there is nowhere for this fluid to go. If this is done to long then the fluid can heat up causing the fluid to break down and causing parts inside the pump to experience metal-to-metal contact. A second reason why whining could occur is because of an increase in the pump speed. The more fluid that is forced through the ports at a time can causes a velocity and the fluid cannot move fast enough. The vanes inside the pump cause a vacuum, which will fill with oil vapor immediately. When the vane reaches the high pressure port a vacuum cell is opened and oil rushes in to fill the void. The third reason when the oil in the reservoir really gets churned up or is low. This will allow a bubble into the pump.