What was found
FLIGHT 1829, BOS-PHL - VERY SOUR SMELL REPORTED COMING INTO CABIN ON TAKEOFF AND CLIMBOUT. SEEMED TO DISSIPATE ON CLIMBOUT. MAINTENANCE CHECKED APU OIL LEVEL, FOUND GOOD. ACCOMPLISHED ENGINE RUN UP AT DIFFERENT SETTING, NO SMELL. ACCOMPLISHED TASK 21-20-00-810-A AND J/C FOR CABIN QUALITY CHECK, UNABLE TO DUPLICATE. REMOVED AND REPLACED RECIRCULATION AIR FILTER, AC IS GOOD FOR SERVICE.