As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PRONOUNCED VIBRATION AT FL290 AND 313 KNOTS. VIBRATION DISAPPEARED BELOW FL280 AND 280 KNOTS. FLIGHT RETURNED TO DEN UNDER AMBER ALERT. *S/D* FOUND LEFT PACK RAM AIR INLET DOOR TORQUE TUBE (213W3130-3) SHAFT SHEARED. REPLACED TORQUE TUBE ASSEMBLY.