What was found
INFLIGHT AFTER TAKEOFF LOUD VIBRATION REPORTED BY FLIGHT ATTENDANT. VIBRATION AT ROW 26 AND AFT. VERIFIED BY FIRST OFFICER. COULD NOT ISOLATE TO SPECIFIC ENGINE OR AIRFRAME. RETURNED TO ORD AND LANDED WITHOUT INCIDENT. FOUND EVIDENCE OF BIRD STRIKE IN THE NUMBER 1 ENGINE INLET. PERFORMED BIRD STRIKE INSPECTION. REPLACED TWO EACH FAN BLADES IN NUMBER 1 ENGING PER WORK CARD 72-91-11. CONDUCTED PAST FAN BLADE REPLACEMENT ENGINE RUN. SYSTEM GROUND CHECKED NORMAL OPERATION.