As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURNED TO DEPARTURE DUE TO NR 1 ENGINE, N1 VIBRATION CLIMBED TO 3.9 DURING CLIMB THRU FL 360-380, REDUCING POWER N1 STABILIZED AT 1.5 IN TURN CLIMBED TO 1.7. TROUBLESHOOT IAW TSM 77-00-00-810-814-A22. VIBRATION DUE TO FAN LUBE AND WITHIN LIMITS FOR QC MONITOR. DEFERRED TO QC MONITOR FOR 10 FLIGHT CYCLES.