As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT CRUISE (FL360 M .78 85.9% N1) LEFT DUCT PRESSURE DROPS BY ABOUT 8 PSI EVERY FEW MINUTES (2 TO 8 MINS) WITH FLUCTUATIONS IN CABIN RATE OF CLIMB AND FLIGHT DECK AIR NOISE. DUCT PRESSURE FLUCTUATIONS WERE LESS FREQUENT AT FL 380 (M .79 89.2% N1) BUT STILL OCCURRED. CABIN ALTITUDE AND DIFFERENTIAL REMAINED STABLE. GROUNDED - YES. REMOVED AND REPLACED PRSOV PER B737-900 AMM 36-11-04. OPS CHECK NORMAL THIS TIME.