Fuel tank vacuum??

Well finally got the engine swapped out in this 1995 TransSport for a rebuilt 3800. Much better..for now anyhow... but now whenever I go to fuel up I hear a "sucking" sound coming from the tank as soon as I open up the filler cap. Why? I mean this has been a problem since we got the stupid thing 2 weeks ago. It has always made this noise but when its on it's last 20L or so of fuel it seems to make the fuel pump work harder (we can hear the thing whining). as soon as we release the cap the sound's gone. What could this possibly be caused from. Any insight would be greatly appreciated.