A B737-700 IN CRUISE AT FL390 AT MIDPOINT TRANS PACIFIC HAS APU EGT SPIKE TO 660 DEGS AND FLUCTUATE. APU SHUTDOWN. FLT DIVERTED.

Narrative

ABOUT 1 1/2 HRS AFTER TKOF FROM ZZZ; THE AFT FLT ATTENDANTS RPTED AN INTERMITTENT RUMBLING NOISE IN THE AFT GALLEY AREA. I SENT THE FO TO THE AFT GALLEY AREA AND HE CONFIRMED A RUMBLING; GRINDING SOUND. THE APU EGT WAS NORMAL. HOWEVER; THE APU CPS FREQ INDICATOR WAS BLANK AND THE AC VOLTS INDICATED 1. THE FLT ATTENDANTS WERE ASKED TO MONITOR THE SOUND AND LET ME KNOW IF IT CHANGED. I CALLED ZZZ MAINT VIA AN HF PATCH AND RPTED THE NOISE AND APU INDICATIONS. I WAS ADVISED IT WAS OK TO CONTINUE ON TO ZZZ1. JUST AFTER ETP [MID POINT TRANS-PACIFIC]; THE APU EGT SPIKED TO 660 DEGS AND BEGAN TO FLUCTUATE. AT THE SAME TIME THE AFT FLT ATTENDANTS CALLED TO RPT A LOUDER NOISE IN THE AFT GALLEY AREA. I SHUT THE APU DOWN AND ADVISED ZZZ MAINT. JUST BEFORE ZZZZZ ON THE X TRACK; DISPATCH CALLED VIA A PATCH AND REQUESTED WE DIVERT TO ZZZ2 FOR MAINT REASONS. WE OBTAINED A CLRNC AT X DIRECT ZZZZZ1 DIRECT OSI DIRECT ZZZ2. WE LANDED WITHOUT INCIDENT; DID NOT DECLARE AN EMER; NOR DID WE REQUEST PRIORITY HANDLING. WE CALLED ZZZ2 100 MI OUT TO LET THEM KNOW OUR ETA. UPON ARR AT OAK; WE CHANGED ACFT AND THE PAX WERE IMMEDIATELY REBOARDED FOR ZZZ1.