Fuel tank

Thanks in advance for any time taken to answer. In my vehicle, every, and I do mean every time I open the fuel tank there is either a big sucking sound or a woosh as it releases pressure (not sure how to check which is it). Do they normally operate like this or is my fuel cap or tank breather blocked/faulty?

The valve on the cap seems top be mobile to the touch and I have no idea where or how to service the tank breather. I would just roll with it but, if I run it for a few miles it runs grainy and seems to struggle, slightly, but if I open the tank before setting off it's smooth as a dream (I might be imagining it).

I have loads of questions for this vehicle. If there are any Trooper Ninja's out there who would like to flex their experience?