so i noticed everytime i turn my car off there is a loud hissing from the Bac area on my s4 n/a. i can't seem to pinpoint it out but i will make a video later, what can it be?

 

I suspect it is a coolant line...
I used to hear hissing around my heater core line (on the firewall) with the car off after a drive, but would never notice anything dripping. However while driving, my coolant light would come on! It could be a slow coolant leak on a loose/leaky line.