What was found
AIRCRAFT WAS GROUNDED: FLIGHT ATTENDANTS REPORTED DIRTY SOCK SMELL SHORTLY AFTER TAKE OFF AND DISSIPATING RAPIDLY BUT RETURNING FROM DESCENT UNTIL LANDING. MAINTENANCE FOUND APU TO BE SOURCE OF ODOR. MAINTENANCE REPORTS APU IS CLEAN AND DRY, NO LEAKS FOUND , NO OIL COMING FROM WITNESS DRAIN. APU WAS INSTALLED MAY 18. RAN APU AND #1 , #2 PACKS WITH BLEED AIR, NO SMELL NOTED. APU RETURNED TO SERVICE.