The power steering on cars is controlled by a pump located in the engine bay. A variety of causes can make the pump squeal or whine.
Insufficient Hydraulic Fluid
Power steering pumps are lubricated by hydraulic fluid which is contained in a reservoir in the engine bay. If the amount of fluid in the reservoir drops below the minimum level, the power steering pump will not be sufficiently lubricated, causing it to make noise. The fluid should be checked by removing the lid of the reservoir and checking the level so the appropriate amount of fluid can be added.
Power steering pumps will often whine or squeal when the temperature is cold and the pump has not warmed up. This will be most noticeable when the car is first started and will usually get worse when the pump has to work to turn the wheel.
Bad or Worn Pump
A bad or worn power steering pump may also make noise when the vehicle is driven, particularly around corners. If the pump makes noise at all times, even when the fluid is topped up, it likely needs to be replaced.