There are some evaporation system components behind the LH rear wheel cover. The purge solenoid in the enginebay will create vacuum to the evap canister and then it will create the vacuum above the fuel in the tank. Probably some hose is loose or cracked. The sound is probably not there when the engine is cold? Because at that time the evap solenoid is not activated and there will be no vacuum present. The evap solenoid is pulsing when it works (the signal comes from the PCM) and that is causing the sound.
The code you have has to do with the leak detection pump, also in the evaporative system. With this pump a leak in the system can be detected. So having the code and hearing a hising sound both point to a vacuum leak behind the rear wheel cover.
JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.