What was found
AIR TURNBACK, DURING CLIMB, AIRCRAFT HAD A LOUD THUMPING NOISE AND VIBRATION MOSTLY HEARD IN AT THE AFT SECTION. PERFORMED AIRFRAME VIBRATION CONDITION INSPECTION. NO DEFECTS NOTED. POSSIBLY APU COMPRESSOR STALL. DURING RON, REMOVED AND REPLACED APU ANTI-SURGE BLEED CONTROL VALVE PER MM.