Gas tanks have the EVAP system that uses engine vacuum to suck air(gas fumes) out of the gas tank thru a charcoal filter and into the engine to be burned.
I hear that sucking sound every time I get gas, and it is air being sucked in, not air escaping.
So it is normal, but only after a longer drive, if you just start up truck and drive a few miles to gas station then you probably wouldn't hear any thing because not enough time has passed to build up enough negative pressure in the gas tank.
Computer uses a Solenoid on a vacuum line to put vacuum to the charcoal filter which then sucks air out of gas tank, this solenoid could be stuck OPEN so vacuum is always being applied to gas tank, this solenoid usually fails CLOSED but could fail OPEN.
EVAP system is an emissions system, so if there is a problem with it you would probably get a CEL(check engine light) and an EVAP code.
Now there are a couple of vents on the system, one on the top of the gas tank and one on the charcoal filter in the engine bay, since you are in Puerto Rico there are a few bugs there, lol, and while it is a long shot a vent could be clogged since you never noticed the sound before.
This should cause CEL to come on and an EVAP code to be set.
Google: check evap system vents
Last edited by RonD; 09-16-2016 at 10:15 AM.