OM states; Some pressure release when fuel cap opened "is normal". But this is just a momentary hiss, as gas cap released.
Hiss for longer periods, is mostly due to Charcoal Canister (CC) being saturated/flooded. This is most common, in models where CC is in the rear.
Once CC gets, heavily saturated with fuel. We'll often hear, see and or smell fumes in daily driving. Over time, the trapped raw fuel, becomes rancid. Rancid fumes, then smell like gasoline that has sat in gas can for many years. Heat (OAT) and ascending mtn passes, will make more pronounced. But when CC really, badly saturated, even cool days without change in altitude, they smell and hiss. Engine, will even have trouble idling.
CC due become saturated, with use. But that could be a million miles or more. If vehicle always properly maintained.
We must look for what caused the CC to become flooded and correct, before R&R of CC. Or we'll damage the new CC.