What was found
2022/01/21 B-18725 CI-5321 JFK-ANC/JFK ENG 4 N1/N2 100%/90% TWO TIMES DURING CLIMB AND A/C VIBRATION FEEL. FLIGHT CREW FELT AIRCRAFT YAW AND VIBRATION CAUSED BY POWER LOSS. AFTER CHECKING OTHER PARAMETERS, THERE WAS NO ABNORMALITY. THE FLIGHT CREW FOLLOW QRH FORWARD & RETARD THE THRUST LEVER TWICE, BUT THE SAME SITUATION OCCURRED. DURING THE THREE TIMES TRIAL PER QRH, THERE WAS NO SOUND OF BANG FROM ENG, AND THE CREW DECIDED TO RETURN TO JFK.