I have a 99 Explorer Sport. When I am about to start my engine I do not hear the fuel tank pressurizing sound that usually hear when I turn the key to the run position, before I crank it. I normally can hear a distinct pressurizing sound coming from the rear of the vehicle, but I no longer can here that sound. Sometimes if I do hear it the sound, it is very faint. What is my Ex's problem?

 

Any ideas about this problem? I need some help.