As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LOW PITCH VIBRATION HEARD CYCLING ON AND OFF IN FLIGHT COMING FROM AIR CYCLE MACHINE AREA. REMOVED AND REPLACED COUPLING AT NUMBER TWO INLET TO DUAL TEMPERATURE CONTROL VALVE IAW EMB AMM 21-51-03. OPERATIONAL CHECK GOOD, RETURNED AIRCRAFT TO SERVICE.