I guess it might be a feature since the charcoal filter gives a bit of resistance when you try to push air through it. Since the fuel tank has some volume it may need to build some gas fumes before the pressure is adequate to overcome the resistance of the charcoal canister. Hence there is some air escaping when removing the fuel cap. At least that's how far my explanation would go.
Since the previous owner removed it and I never ran a charcoal canister before this was somewhat of a new experience.
The fuel expands a bit too in the warmer weather, that would cause it. That's why F1 used to fill at lower temperatures, from memory it was 12-15 degrees, the fuel was more dense and compact and took less time to fill compared to the same amount at 25 degrees because off expansion. Then there's the tank return line that would build pressure too
It's my understanding any excess fumes from the tank are vented to atmosphere via the charcoal filter. The charcoal removes any dangerous elements from the air and recirculates them in the intake. One of the reasons I've retained mine as the cars can sit for a while with the bonnets closed. Without the filter there's a possibility of a build up of fumes in the bay. Unlikely but there's a potential.
The noise I experience isn't pressure, seems more like vacuum?
Vacuum should be vented by the gas cap and charcoal canister. Bit of air released may wel be due to the resistance of the spring in the charcoal canister. I'm not to worried about it anymore thx for the info and help.