There is a noisy relief valve in the hydraulic pump.

There is some noise in all power steering systems.

One of the most common is a hissing sound. The hissing sound is most evident at standstill parking. A hiss is a high frequency noise.

The noise is present in every valve and results from a high velocity fluid passing over the valve orifice edges. There is not a 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 when slowly turning 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 objection. Hobourn pumps are not serviceable. If a Hobourn pump requires disassembly, then replace the pump.