As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
USING APU FOR AIR CON, AIR IS PULSATING THROUGH VENTS IN COCKPIT. FOUND RIGHT RECIRCULATION FAN TO CAUSE VIBRATION. TROUBLESHOT AND FOUND THE RIGHT RECIRCULATION CHECK VALVE BAD. REMOVED AND REPLACED CHECK VALVE AND RECIRCULATION FILTER AS PER AMM. LEAKS AND OPS CHECKED GOOD NO PULSATING NOTED IN COCKPIT.