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Strange hiss from around gas tank

After 3 hours of trying to find the source of a sound in the garage, I found it coming from in or near the gas tank of our '08 Hyundai Entourage. It only happens in extreme cold after the vehicle has sat for a while. It has the sound of either air release or a faint motor running…the sound is like being on the other side of a wall from where a faucet is run. It lasts from 3-10 seconds…and intervals between can be from 30 seconds - 2 minutes. The best I can come up with is the cannister from the vapor recovery system is somehow turning on and off, perhaps from the extreme cold shrinking metal and gas or whatever. I am certainly no expert, but spoke to two different Hundai technicians who haven’t a clue and thought my idea sounded as good as anything…that’s a lot of help. Any ideas?

You could try a new gas cap. It is cheap easy and a possibility.

The first step is to read the Owner’s Manual. In addition to the hundreds of other general reasons for this recommendation, I have a specific reason for recommending it, namely the possibility that what you are hearing is perfectly normal.

As an example of what I am talking about, several months ago, I was reading the manual for my friend’s ‘08 RAV-4 (yes, I read other peoples’ manuals, in addition to the one for my car, in an effort to learn as much as possible). In the RAV’s manual, there was information regarding a self-test that the vehicle’s evaporative emissions system performs under certain conditions. While I cannot recall the exact description, I do remember that the manual stated that, after the vehicle has been sitting for several hours, the owner may hear the sound of an electric motor for a minute or two, somewhere underneath the car in the general vicinity of the driver’s seat. The manual states that the noise may sometimes be more noticeable than at other times, and that it is nothing to worry about as it is a normal diagnostic test that the vehicle is programmed to undergo at certain times.

Is this the same phenomenon that the OP is hearing?
I don’t know the answer to that question. And, it is entirely possible the many Hyundai techs also do not know the answer to the question.

I would suggest first reading the manual and if that does not answer the question satisfactorily, then I have two other suggestions:

Contact Hyundai’s corporate-level Customer Service staff (contact info is contained in that same very useful Owner’s Manual!) and ask to speak with a technical specialist regarding the problem.
Request an inspection of the car by Hyundai’s Regional Technical Representative. That same manual will give you the contact info for this person.

Thank you for the feedback thus far. To continue the discussion, I have spoken with two Hyundai techs thus far (one being my daughter’s finace). Neither could come up with any ideas. We tried the gas cap idea with no result. The manual gives little explanation, but it did help me suggest to the techs that it had something to do with either the emission control canister or the OVRS (Onboard Vapor Recovery System). The canister was eliminated because the sound would have then come from about a foot in front of the gas tank. They both believe the OVRS is either working improperly or actually working too well, meaning too often.

Both techs have now made contact with Hyundai Coporate and have more specific schematics. thus far, they have received or deduced no definitive answer.
Any more helpo here that I can pass on would be appreciated.