As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE AT FL340 AN ABRUPT VIBRATION WAS FELT AFTER SLOWING, REALIZED WAS COMING FROM LEFT ENGINE, VIBRATION WAS AS HIGH AS 2.0. THE AIRCRAFT WAS GROUNDED. ACCOMPLISHED A NR 1 ENGINE VIBRATION CHECK IAW AMM 71-00-00-710-009-B-6 AND FAN VIBRATION LESS THAN 4 UNITS WITH NOISE AND RUMBLE TSM 77-00-00-810-862A. VIBRATION MAXED OUT AT 70% NR 1 TO 3.7 NR 1. OKAY FOR SERVICE.