ACFT EQUIP PROB RECIRCULATION FAN 'ATE ITSELF UP.'

Narrative

WHILE CRUISING AT FL190; FLT ATTENDANT RPTED 'HIGH PITCH SQUEAL' ON TOP OF THE CABIN AT SEAT ROW 13. FO WAS SENT BACK TO INVESTIGATE. UPON RETURN; HE DESCRIBED NOISE AS A 'PUMP ABOUT TO BURN ITSELF UP.' ALL COCKPIT INDICATIONS WERE NORMAL -- WE TRIED TURNING OFF RECIRCULATION FANS AND ADJUSTING COCKPIT TEMPS (ICE IN DUCTS) IN ORDER TO STOP THE NOISE TO NO AVAIL. NOISE WAS RPTED COMING FROM ON TOP OF THE CABIN WHERE NO PUMPS EXIST. WE REVIEWED CHKLISTS AND ACFT OPERATING MANUAL TO SEE IF A SOLUTION COULD BE FOUND. DURING THIS TIME; ATTENDANT CAME TO THE COCKPIT AND SAID THAT A 'HVY ELECTRICAL SMOKE SMELL WAS IN THE CABIN.' WE DECLARED AN EMER AND DIVERTED TO FT SMITH. SMELL WAS UNNOTICEABLE IN THE COCKPIT. AFTER AN UNEVENTFUL APCH AND LNDG; ACFT WAS TAXIED TO THE TERMINAL. FLT ATTENDANTS WERE BRIEFED TO PREPARE FOR AN EMER EVAC AND TO NOTIFY ME IMMEDIATELY OF ANY CHANGE IN CONDITIONS IN BACK (IE; VISIBLE SMOKE OR FLAME). AFTER PAX DISEMBARKED ACFT; FIRE AND RESCUE CAME ABOARD ACFT. THEY COULD ALSO SMELL THE ELECTRICAL SMELL; BUT COULD NOT LOCATE THE SOURCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE AT72 FOR AN ACR. A RECIRCULATION FAN IN THE OVERHEAD COMPARTMENT 'ATE ITSELF UP' PRODUCING THE ELECTRICAL SMOKE. THE PAX WERE DISEMBARKED NORMALLY AT THE TERMINAL.