I'm going to try to make this easy to understand. So it's very strange what happens, I have 1987 303 5.0 f150 and it's duel tank. When I switch the lever to the rear the cluster gauge goes to passed full. Doesn't move, so I keep it there for a bit switch back to front tank, and that cluster gauge is going down. Now today I switch to read tank it says half full! It also started making a low humming noise. I filled up the front tank and it's still making that humming noise. What is going on??

 

Try again.

 

x2

 

x3 After that check the switch, then the wiring - look for breaks, corrosion, tape jobs etc.

 

x4. The 'reading full' issue is probably because the sensor is grounded out. I don't know about any humming noises other than regular fuel pump whine.