A B737-300 CREW ADVISES GND CTL THAT THEY MUST RETURN TO THE GATE TO INVESTIGATE A SUSPICIOUS SOUND FROM THE AFT OF THE ACFT.

Narrative

DURING TAXI-OUT IN CLE; THE 'C' FLT ATTENDANT INFORMED THE CREW THAT SHE WAS HEARING A 'BANGING' NOISE FROM THE AFT AREA OF THE ACFT. THE CREW HEARD NOTHING UNUSUAL NOR WERE THERE ANY ACFT SYS ANOMALIES NOTED FROM THE FLT DECK. I MADE A DECISION TO RETURN TO THE GATE HOWEVER; AS A PRECAUTION. I TAXIED TO THE HOLDING AREA AT THE APCH END OF RWY 5 AND ADVISED ATC THAT WE NEEDED TO TAXI BACK TO THE GATE. AS SOON AS WE ADVISED ATC OF THE GATE RETURN HE ADVISED ME THAT THERE WAS A FIRE FROM THE APU TAILPIPE. I IMMEDIATELY REQUESTED THE FIRE EQUIP AND SENT THE FO TO THE REAR OF THE ACFT TO CHK FOR FIRE INDICATIONS. THERE WAS NO LIGHT IN THE APU FIRE HANDLE NOR WAS THERE A FIRE BELL. HOWEVER; I PULLED THE APU FIRE HANDLE; FIRED THE BOTTLE AND TURNED THE APU SWITCH OFF AS A PRECAUTION. THE FIRE DEPT WAS ON SCENE WITHIN 2 MINS AND THEY ADVISED ME THAT NO EVIDENCE OF A FIRE COULD BE SEEN INSIDE OR OUTSIDE OF THE APU COMPARTMENT. BOTH MAIN ENGS WERE SHUT DOWN AT THE REQUEST OF THE FIRE DEPT. THE ACFT WAS THEN TOWED BACK TO THE GATE BY MECHS WHO INSPECTED THE APU PRIOR TO TOWING THE ACFT AND AT THE GATE. MAINT CONFIRMED THAT NO APU FIRE HAD OCCURRED. THE APU HAD 'TORCHED' AS A PROBABLE RESULT OF A MALFUNCTIONING FUEL CTL UNIT. MAINT PLACED THE APU ON MEL STATUS AND THE FLT WAS DISPATCHED AND DEPARTED NORMALLY.