What was found
AVNCS FAN OVRD, ON TAKEOFF. CABIN INFLO LOW. EXTREMELY LOUD. BARELY ABLE TO COMMUNICATE. STAYED BELOW 10,000 . RTB. REF FIM 21-26-04 AND 21-31-01-00, CABIN HAVING TROUBLE PRESSURIZING. NO FAULTS NOTED IN FAULT REVIEW. FOUND F/O SLIDING WINDOW OPEN AT REAR, WHILE LATCHED AT THE FRONT. FULLY OPENED AND CLOSED WINDOW WITH NO ISSUES OR BINDING. AIRCRAFT OKAY FOR SERVICE.