All hydraulic dampers make noise when they are compressed or extended. Even brand new ones straight out of the box. The hissing sound is the oil and gas passing through small internal orifices when the fluids transfers from one chamber to another as the piston moves.
Sometimes you hear it more easily because you have the radio off or the window open or you are driving next to a fence or building that reflects the sound back to you.
Hearing the shocks or struts is not indicative of a problem (in fact, if all the fluid has leaked out, the shock will actually be quieter). Finding leakage, seepage or experiencing a tire 'hopping' as the car is driven is an indicator of a failed shock. The old 'bounce' test is not a reliable way to check the condition of the dampers.