Squatting Dog
FRF Addict
The government will not allow the fuel vapor to vent to atmosphere anymore so, The pressure that gets built up in the fuel system must go somewhere. When you stick the nozzle in to fill up the vapor must go into the canister so it can be captured. The canister vent control opens and lets the vapor into the canister... I think the noise you are hearing is the release of pressure not the sound of a vacuum...
I am thinking you are correct, and the fact that it happened when the tank was near empty. Means there was a more room for gas vapor/pressure. Plus, if the pump is in the tank, an near empty tank would expose the pump, creating more noise..
-Greg