Narrative:

About 1 1/2 hours after takeoff from ZZZ; the aft flight attendants reported an intermittent rumbling noise in the aft galley area. I sent the first officer to the aft galley area and he confirmed a rumbling; grinding sound. The APU egt was normal. However; the APU cps frequency indicator was blank and the AC volts indicated 1. The flight attendants were asked to monitor the sound and let me know if it changed. I called ZZZ maintenance via an HF patch and reported 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 degrees and began to fluctuate. At the same time the aft flight attendants called to report a louder noise in the aft galley area. I shut the APU down and advised ZZZ maintenance. Just before zzzzz on the X track; dispatch called via a patch and requested we divert to ZZZ2 for maintenance reasons. We obtained a clearance at X direct ZZZZZ1 direct osi direct ZZZ2. We landed without incident; did not declare an emergency; nor did we request priority handling. We called ZZZ2 100 mi out to let them know our ETA. Upon arrival at oak; we changed aircraft and the passenger were immediately reboarded for ZZZ1.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.