Sucking sound from gas tank?

I have a 2011 Prius, only owned it a couple of months. Today while it was idling I noticed a sucking sound coming from the gas tank. Not constant, sort of a second of suction then a second stopped, and repeat. Thought maybe the gas cap wasn't on right, opened it and closed it again, then finished driving where I was going to monitor it. The sound continued until I turned the car off. Went to try it later in the day and there was no sound, but I don't exactly trust that it's a non-problem yet.

Has anyone encountered this or have ideas about what the problem could be? A quick Google has not turned up much, though I could be using the wrong terms.

 

Thank you for the warm welcome!

Bisco - I had 3 fuel bars, about as low as I like to let it go. Interesting theory, that fuel quantity could be tied to it. I will see if I can get a recording of it, if it keeps happening.

WilDavis - Heh, yes, it does make some different sounds that my Subaru or VW never did.

Sillylilwabbit - I thought the same thing about the gas cap not being on right, but re-doing that didn't seem to help. A new gas cap is a rather small investment, would be nice if that was all it was!

 

The phone is also good for posting when you are stuck in waiting rooms seemingly forever.

Update: The sound has not repeated, but now the check engine light is on. Possibly unrelated, though that would be quite a coincidence. Earliest I can get in to get the code read is Monday.

I gassed it up and while I was at it flipped the rubber gasket per DRACO's suggestion - didn't make the sensor light magically disappear, but it doesn't hurt.

The temperatures here are fairly cold, below freezing at night and 30s, maybe low 40s during the day. I've known sudden cold snaps to trip the tire pressure sensor on several cars (including this one).