What was found
DURING FLIGHT 3385 FROM MSP, RT INTAKE CAUTION LIGHT ILLUMINATED EN ROUTE TO BISMARCK AT 16,000 FT, CYCLED ON 15 SECOND INTERVALS. AIR RETURN TO MSP. MAINTENANCE REMOVED AND REPLACED LOWER INLET DUCT P/N NH24668-169, S/N ON L155349, S/N OFF L291266 IAW 71-60-05-4. OPS CHECK GOOD.