As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AN ENGINE VIBRATION WAS NOTICED BY CREWS IN FLIGHT AT 50-52 PERCENT N1. MFG ADVISED THAT THIS IS NORMAL AND CAUSED BY A BLEED AIR VALVES TRANSITIONING. ON 10/10/2008 TWO BLEED AIR VALVES WERE REMOVED AND REPLACED. THERE HAS NOT BEEN AN ISSUE SINCE.