There was no backup alarm on those from the factory. You may be hearing the fuel pump, especially if you don't hear it inside with the windows closed. Stand in back and listen when a helper runs the ignition switch. The fuel pump will run for only one second after the ignition switch is turned to "run", then it will turn off until the engine is rotating, (cranking or running). If that's what you hear, it's the fuel pump. Mine in my '88 Grand Caravan has been doing that for over five years. I can hear it outside easily but not inside.