Looks like a “bingo” here. Hooray, hammerbach!
*Are you talking about the fuel cap?!
Whenever I unscrew it to pour in some more fuel, a HEAP of air gets sucked in (out?), or “WHOOOOOSSH” as you like to describe it. I just assumed this was normal?!*
No. A little bit of “whooshing” could be considered normal, a lot indicates a significant vacuum has developed in the tank. Replace the fuel cap and the whooshing as well as your problem should be gone. You can first try leaving the cap loose, as suggested, to be certain.