What was found
AT INITIAL LEVEL OFF AT 350 NOTICED VIBRATION THROUGHOUT COCKPIT. AFTER T/S PROBLEM WAS DETERMINED TO ONLY OCCUR WHEN RT N1 RPM WAS BETWEEN 82 AND 83 PERCENT. PROBLEM VIBRATION WAS CONTINOUS WITH RT N1 BETWEEN 82 AND 83 REGARDLESS OF LT ENG SETTING. RT EPR APPROX 1.8 EGT 430 N2 87.5 FF 2900. C/W BOROSCOPE OF 6TH, 7TH AND 13TH STAGE COMP BLADES 1ST STG HPT BLADES AND 1ST AND 2ND STG LPT BLADES. NO DISCREPANCIES. CHECKED MAIN OIL FILTER AND RAN BOTH ENGS AT DIFFERENT PWR SETTINGS SEVERAL TIMES. NO PROBLEMS OF VIBRATIONS NOTED.