What was found
UPON STARTING THE APU & SELECTING THE BLEED AIR ON, THE FA'S REPORTED A STRONG NATURAL GAS SMELL IN THE CABIN. THE RAMP THEN REPORTED A STRONG GAS SMELL IN THE AFT CARGO PIT. UPON INSPECTION, THE SAME SMELL WAS NOTED WITH SMELL GETTING WORSE TOWARDS AFT OF THE A/C. ( APU BLEED AIR SYSTEM INOP ). REMOVED FROM SERVICE. PERFORMED REMOVAL OF ODORS FROM AIR CONDITIONING DUCTS. REF -8 400 AMM TASK 21-00-00-160-801 PARA 4B. NO UNUSUAL ODOR IN AFT CABIN OR AFT PIT NOTED. AIRCRAFT RETURNED TO SERVICE.