As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CLIMBING THROUGH FL 300, A HIGH INTENSE SQUEALING WHISTLE BEGAN, CREW HAD TO SHOUT TO HEAR EACH OTHER. INTENSITY INCREASED DURING CLIMB UNTIL IT BECAME PAINFUL. CAPTAIN INITIATED DESCENT. SOUND WENT AT CABIN ALTITUDE OF 3600 AND CABIN DIFFERENTIAL PRESSURE OF 6.0. MAINTENANCE INSPECTED AND FOUND THE NR 2 WINDOW SEAL DAMAGED. REPLACED WINDOW SEAL IAW 56-11-13 AND 05-51-24 .