Normal.
Modern fuel systems are predominantly sealed systems, fumes aren't supposed to be released to the atmosphere, so in most cases the vapours get channeled to the EVAP charcoal canister. Then these get used when the car is in motion.
Most of the time the system will run at less than atmospheric, so the hissing noise will indicate a vacuum in the tank, but on occasion (depending on external ambient temperature, pressure etc) it can be under positive pressure, but this is rarer.
(Not all parts of the fuel system is under pressure, just the fuel rail and injector system is under high pressure. The rest of the system is "pumped" as opposed to "high pressure")