If original, a 12yr old starter can be suspect and tapping may or may not be definitive of starter reliability as no one can predict when starter brushes wear out; the main reason for starter failure after years of reliable use. However, not seeing 12v on the main battery cable to the starter may be the problem. Solve why 12v is missing on the starter end of this cable and you may restore proper function to the starter.
Examine both battery cables for corrosion, powdery deposits on cable ends connected to both battery terminals. A warm solution of water and baking soda will dissolve dried battery acid with a plain water rinse. Bubbling is the reaction to neutralizing sulfuric acid. Do not allow this solution into the battery cells. Ensure connections are tight. Next are ground connections; to chassis and engine block. Last is starter connection from the battery; be sure its secure and not loosely connected. Starters draw the most current from batteries so battery red and main grounds must be clean, free of corrosion and securely connected to allow maximum current flow to the starter.