As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: PILOT DIVERTED TO STL ON ACCOUNT OF UNEXPLAINED VIBRATION. EMERGENCY WAS DECLARED. MAINTENANCE FOUND VIBRATION TO BE FROM RECIRULATION FAN. REMOVED AND REPLACED RECIRCULATION FAN AND OPS CHECKS NORMAL IAW AMM 21-20-12. NO VIBRATION FELT IN CABIN.