What was found
DUE TO A LACK OF FACILITIES FOR 'HIGH-POWER' RUN UP, MX PERSONNEL HAD REQUESTED THAT FLT CREW CONDUCT PWR ASSURANCE RUN (UP TO 1.3 EPR) AT END TO TAXIWAY, PRIOR TO DEPARTURE. REQUEST WAS A FOLLOW UP TO PREVIOUS REPORT OF ENG VIBRATION (MX HAD ALREADY CONDUCTED IDLE PWR RUNS WITH NO FAULT FOUND). DURING PWR ASSURANCE RUN, FLT CREW NOTED 'RUMBLE/SURGE' IN NR 3 ENG. FLT CREW ELECTED TO RETURN TO GATE FOR MX ACTION. A/C RETURNED TO GATE W/O FURTHER INCIDENT. MX INSPECTED ENG FOR COMPRESSOR STALL AND PERFORMED BORESCOPE, NO MALFUNCTION/DAMAGE NOTED. CFDS INDICATED FAULT IN FMU. FMU REPLACED. ADDITIONAL PRECAUTION, NR 3 EEC REPLACED. ENG GROUND RUN WITH NO FAULT FOUND. A/C RETURNED TO SERVICE.