As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: SEVERE SQUEAL FROM L2 WINDOW AT FL390. STARTED AT FL370, 20 MINUTES INTO FLIGHT (RIGHT FORWARD, LOWER PART IS WHERE THE NOISE SEEMED TO COME FROM. AIRCRAFT AIR INTERRUPTED BACK TO DFW. EMERGENCY WAS NOT DECLARED. FOUND L2 WINDOW LOWER FASTENERS BELOW TORQUE VALUE SPECIFIED IN THE MAINTENANCE MANUEL. RE-TORQUED FASTENERS PER AMM 56-11-05.