If it is the 7.3L diesel, it could be air in the lines and oil. They have a hose that equalizes the pressure and it is supposed to make it quieter. This is known as the HPX or High Pressure Crossover. It connects the left and right side high pressure rails together. It gives you more even pressure and less spikes. Oil runs the injectors in the Powerstroke.

Also they have quick disconnects that are great for pressure but not vacuum. These quick disconnects are between the tank and pump, hence the vacuum. People have done things that makes the return line pump the fuel somewhere else in the tank and removed the quick disconnects altogether. AKA Harpooning the tank.

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