As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
30 MIN INTO FLIGHT AT 340 FELT AND HEARD PRESSURIZATION "PULSING". OBSERVED DUCT PRESS ON RIGHT DUCT PULSE BETWEEN 8 AND 25 PSI SPLIT FROM LEFT SIDE. CABIN VS WOULD RANGE +/- 500 FPM CONTINUE FOR ENTIRE FLIGHT 2+ HOURS. AT LOWER ALTITUDE SPLIT IN DUCT PRESSURE INCREASE TO 35. GROUNDED - YES. DIVERSION -- REPLACED BLEED AIR REGULATOR IN REF TO B737-900 AMM 36-11-03. COMPLETED ASA T/C NG-36WC-00001 PERFORMED ENGINE BLEED AIR CROSSOVER OPERATION TEST OPS CHECK GOOD IN REF TO AMM 36-11-00.