Just looking through this part of the forum...so I am a little late in posting this. But since the fuel tank is made of a somewhat flexible polymer, I think the popping from behin the driver may be caused if the tank experiences a vacuum during driving and fuel being "pumped" out and not enough air let back in to replace the fuel.
I know the car needs to have the fuel cap installed when the engine is running or it may flash the Check Engine light according to the manual.
To figure out if this is the cause of it I would consider this:
1. Does it happen after both short and long interval of driving when you guys come to a stop ???
2. Have someone open the fuel cap immediately after you run the engine at slightly increased RPM for a few minutes and listen for a hissing sound (Air rushing in)
3. Or have one of your best friends lay under the car while you run the engine through a couple minutes of RPM cycles at 1/3 full tank and check if he can see the tank walls collaps slightly ;-)