What was found
THE FLIGHT CREW REPORTED THAT AT 1 + 30 MINUTES IN FLIGHT, FL310 THE RIGHT ENGINE FAN VIB WENT TO 4.0. THE CREW OBSERVED THE RIGHT FAN VIB RUN FROM 4.0 TO .3 THEN OSCILLATED 4.0 TO VARIOUS NUMBER. THE OSCILLATIONS LASTED 2-4 MINUTES UPON LANDING, THE RIGHT FAN VIB READ .1. THEY STATED NO ACTUAL ENGINE VIBRATIONS FELT IN FLIGHT. OPERATIONS WERE CONTINUED UNDER MEL 77-30-1. DURING THE MAINTENANCE EVENT THE MAINTENANCE BASE TROUBLESHOT AND DETERMINED THE ENGINE VIBRATION MONITORING SIGNAL CONDITION WAS AT FAULT. A REPAIRED UNIT WAS INSTALLED, OPERATIONAL CHECKS WERE SATISFACTORY AND THE AIRCRAFT WAS RETURNED TO SERVICE.