It could be from a lot of places. Most likely culprit is that your fuel pump is going out. The easiest way to get to them is take the bed off. Its 4 if I remember right bolts. You'll need a T55 socket to fit those bolts and a couple buddies to help lift it off. Otherwise you have a small leak somewhere in the line. But because the line is under pressure it would most likely be a visible leak. The other thing is change the fuel filter. Might as well while you are down there?