Whirring noise from rear of truck, fuel pump?

Hello,

If the fuel gauge is not working correctly it is due to a faulty fuel level sending unit.
The fuel level sending unit consists of a float and a resistor. The float moves up or down with the fuel level and contacts at different part of the resistor, the resistor send that info to the gauge for you to see the level.

In order to replace the level sensor you will have to remove the fuel tank, as the level sensor is a part of the fuel pump assembly.

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump

I have attached diagrams for your reference.

Please let me know of any questions.
Thank you.