Serious pressure every time I open fuel cap

[Winderman69]

Mine hisses all the time, sounds like a distant train whistle. Even after running thru a full fuel load yesterday the cap popped off like a rocket on E.

I would check the vent tube then

 

[snowbeast]

Never had any type of poping off when unscrewing the cap,but sometimes after riding and stopped for refueling I can hear a hiss as I unscrew the cap,but that seems normal to me.