A short hiss as you unscrew a correctly working shock pump is normally air leaving the pump hose, rather than your fork or rear shock emptying itself. RockShox and FOX both describe this behaviour in their setup guidance. The confusion often comes when you reconnect the pump and see a lower reading.
Two different moments, two different effects
When a pump is connected, air from the suspension fills the pump’s hose and gauge. The combined space is larger, so the displayed pressure can be lower than the setting you remember. RockShox explains both the connection drop and the disconnect hiss in its Charger 3 setup guide.
That is why reconnecting immediately is not a clean test of whether air escaped during removal. The act of checking changes the system you are measuring. The size of the change depends on the equipment; there is no universal number of PSI to add as compensation.
Do not chase the hiss with extra pressure
Set pressure using the instructions for your exact fork or shock, keep within its stated limits and disconnect the pump using the pump maker’s procedure. Do not add an arbitrary extra amount merely because you expect a hiss.
FOX’s shock-pump guidance also distinguishes air escaping from the hose from air leaving the shock. The principle is useful, but the pressure figures in an older manual are not settings for a modern model. Always use the correct current manual for your suspension.
Air-chamber equalisation is separate
Some air-spring setup procedures require cycling the suspension to equalise its chambers, followed by another pressure check. Follow the model-specific sequence rather than copying a procedure from a different fork or shock. A pressure change during that process has a different explanation from the brief noise made when removing the hose.
When should you investigate?
A brief disconnect hiss alone is weak evidence of a fault. A valve that continues leaking after the pump is removed, suspension that loses support between rides, or a marked change in sag deserves investigation. Stop riding if the suspension is no longer working normally and have the valve, pump connection and suspension checked by a competent workshop.
For useful records, note the suspension model, pump used, final setup pressure and sag. Compare behaviour under similar conditions and follow the same setup process each time. A sequence of reconnect-and-read measurements can otherwise make a normal system look as though it is losing pressure continuously.
The practical answer: set up by the correct manual and judge the suspension’s behaviour as well as the gauge. A hiss is a sound; it is not, by itself, a diagnosis.