Cadillac Repair: Fuel Gauge, fuel gauge, plastic pan

Question
All of a sudden my fuel gauge is acting up. I filled my tank up but every 20 seconds or go.  The needle will fluctuate off full rotate toward E and then wind up in the correct spot.  The trouble is every time it does it an anying sound (like you seat belt is not on) goes off.  What is causing this and how can it be fixed.  Happened out of the blue after I filled my tank.

Answer
Need YEAR-MODEL-ENGINE SIZE of vehicle. Sounds like the fuel sender in the gas tank is faulty and when it gets to empty the alarm goes off to tell you that you are low on fuel.

Also inside the gas tank there is a plastic approx 12" square plastic pan that prevents the fuel sending unit from going back andd forth from full to empty due to gas sloshing around in the gas tank and sometimes this plastic pan comes loose and floats around in the gas tank hitting the gas sending unit and causing this type of problem.

Must remove gas tank and remove sending unit and pump from top of gas tank and reach hand down in the hole and feel if this pan is loose. If it is, replace the gas tank. If the pan is not loose, replace the gas sending unit and I would also suggest replacing the fuel pump as well.