Hissing Coming From Fuel Tank Area

Okay, so I've got another problem on my hands. I started up my fox earlier and a hissing sound is coming from the rear end. I can't tell exactly where it's coming from.
The sound sounds like a leaking tire. What brought it to my attention was the fuel pump working noticeably "harder" than usual. It literally sounds like a small generator (humming).

Its a stock fuel pump, lines, etc.

It's an on and off thing. The pump does hiss only when the car is running. I've inspected all of my connections and fuel lines. It is not leaking fuel. I've removed the gas cap but it still continues to hiss.