As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLT 923 BOS-ORD, AIR RETURN TO BOS DUE TO ABNORMAL VIBRATION AT V1 4.1 ON NR 2 ENGINE. WENT BACK TO NORMAL AFTER TAKEOFF, BACK TO 4.2 ON CLIMB AND RETURNED NORMAL AFTER POWER WAS REDUCED. THE TECHNICIAN PERFORMED VISUAL INSPECTION OF FAN AND LPT BLADES WITH NO FAULTS NOTED. FAN LUBE AND TRIM BALANCE WERE ALSO ACCOMPLISHED AS PER EMM 72-00-00.