I removed the panel to see if the gas tank was deformed due to a suspected pluged vent. I have been hearing suctiion when removing the gas cap. This is going from full to half tank then filling up. I also hear a gurgling for about twenty seconds after starting. The sound is coming from the tank, the pumps sound fine.

I noticed the white can next to the tank and it was unpluged. What is this thing?

__________________
1986 560 SEL
1960 Ranchero
1970 Cougar XR7
2002 Crown Vic Sport
1992 F150