What was found
DFW - AIR TURN BACK - FLT 421 - AFTER DEPARTURE CREW REPORTED LOUD SQUEAL COMING FROM CAPTAIN'S WINDOW. AIRCRAFT RETURNED TO DFW AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE CLEANED AND LUBRICATED THE CAPTAIN'S SLIDING WINDOW SEAL. ALSO, RE-SEALED THE CAPTAIN'S FORWARD, L1, WINDOW. PRESSURIZATION CHECKS WERE GOOD.