As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURNBACK -- DURING CLIMB AT FL 21.0 A LOUD NOSE WAS COMING FROM CAPTAIN SLIDING WINDOW. PRESSURIZATION CABIN RATE OF CLIMB INCREASED FROM 500 FPM TO 400 FPM - STABLE DURING CRUISE FLT. NOISE VERY LOUD - HARD TO HEAR RADIO'S. REMOVED AND REPLACED CAPTS SLIDING WINDOW IAW MM.