What was found
AIRCRAFT 3801, DURING PUSHBACK, A BURNING RUBBER SMELL DEVELOPED IN THE AFT GALLEY. MAINT CONTROL THRU RADIO HAS US RETURNED TO GATE. NO ODORS NOTED AT THIS TIME. C/W NO GO ODI PREVENTION 2172 AND W/C 0550-9001-B737. NO TROUBLE FOUND. INSPECTED APU INLET AND REMOVED AND INSPECTED APU PNEU DUCT. NO TROUBLE FOUND. REINSTALLED DUCT. REPLACED BOTH CABIN AIR RECIRC FILTERS REF B737 AMM 21-25. INSPECTED AFT CABIN DOOR HEATER AREA. NO TROUBLE FOUND. RAN PACKS FOR 1 HOUR. NO ODORS PRESENT.