Gas tank bubbling?

Hi there! While trying to figure out what's up with my car, I noticed that while the car is running, if I open the gas cap, I can hear the gas bubbling! It sounds like one of the aquarium air things. Anyway, it seems very very weird, and I am wondering if it is normal? If not, what could be wrong? My dad's cars and his friend's car don't do it.

Thanks in advance!