The vented caps vent to outside air for pressure and vacum. When hot fuel cools it will pull a strong vacum so you have to vent under a vacum and with pressure when the fuel heats up and exspands. The newer cars that are sealed caps still release pressure and vacum at certain pressure and vacum settings as a safety feature in case the vent lines get blocked. I would replace the gas cap first with a good vented cap and see if that does it. Make sure it is a vented cap as I have seen guys think they got a vented cap and did not. Its not unusual to hear a little noise from a non-vented cap when it releases past its safety setting but vented caps dont usually make any noise because it is always vented and should not build pressure or vacum. Ron