What was found
(CAN) CREW REPORTED 3. MILS VIBRATION AT DECENT AT 70 PERCENT. LATER A LOUD BUZZ AT CRUISE, AND VIBRATION WAS 1.8 MILS. BOROSCOPED ENGINE, A GROUND RUN VIBRATION WAS WITHIN LIMITS. A ONE HOUR FLT AND A BUZZING COULD BE HEARD FROM A/C. AC/DC GENERATORS & HYDRAULIC PUMPS FROM LT ENGINE INSTALLED ON RT ENGINE WITH NO CHANGE. VIBRATION SURVEY IAW MAINT MANUAL CARRIED OUT AND RESULTS POINTED OUT TO HP COMPRESSOR. ENGINE REVIEW AT RRC CONFIRMED VIBRATION. ENGINE STRIP REVEALED HP COMPRESSOR ROTOR STAGE 11 TO 12 FAILED. SPACER HAS A TEAR APPROX 6 INCHES AND HEAVY RUB.