I have a 1983 Mercedes diesel. It has just started making a funny noise when I turn the key on but before I actually start the engine. It seems to be coming from the middle of the car, kind of just behind the driver's seat. Some people have told it's the fuel pump, while others have told me it's an extra fuel pump. They have also said that it could run for another year before the pump actually quits. What IS making his noise?