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.
A growling sound is not normal. Growling is normally caused by low fluid in the power steering reservoir, or air in the system.
Fluid level should be checked. If it is low, then inspecting for a leak is needed. Areas of leaks can be from the power steering pump itself, pressure or return lines or the steering gear or rack and pinion. After the leak is repaired, the system must purged of air. This is known as "Bleeding the System". If this is not done, the nosie will continue, and you may think the problem is not fixed, when it actually is.