What was found
AIR INTERRUPT, RETURNED, EMER NOT DECLARED. LANDING UNEVENTFUL & ACFT NOT OVERWEIGHT. DURING CLIMB, BEGAN FEELING CABIN PRESS FLUCTUATIONS ACCOMPANIED BY LOUD SQUEAL FROM RT AFT OF ACFT THAT INCREASED WITH ALTITUDE. R & R OUTFLOW VLV ASSEMBLY IAW AMM 21-31-51. ACCOMPLISHED OPERATIONAL TEST IAW AMM 21-31-00. SYS CHECKED NORMAL.