There is a noisy relief valve in the hydraulic pump.

There is some noise in all power steering systems. One common noise is a hissing sound. A hiss is a high frequency noise. The hissing sound is most evident at standstill parking. The noise is present in every power steering valve. The noise is the result of a high velocity fluid passing over the valve orifice edges. There is no relationship between the hissing noise and the performance of the steering. A hiss may be expected when the steering wheel is at the end of travel, or turning slowly at a standstill. Do not replace the valve unless the hiss is extremely objectionable. A replacement valve will also exhibit some slight noise and is not always a cure for the hissing sound.