My 4x4 Sapphire never misses a beat when starting yet tonight it didnt want to start. The engine turned over but never fired. Am i right in thinking the fuel pump makes a buzzing sound when ignition is on, but mine made more of a humming sound?? I tapped the pump a few times and eventually it fired. When i got home i turned it off and on again, although it fired then pump just before firing buzzed then when silent..... suggestions on service/ repair/ replace............PLEASE HELP