Aural Warning in cabin when transferring power

[xiaok]

Crew report there is an occasional alarm bell(seem like fasten seat belt) in the rear cabin when transferring power from ENG to APU.

Anyone experienced similar situation?Thanks in advance, I appreciate for your kind infos.

[tweakjr]

Possible bad R9, SPCU, or M1474(Int sys accy unit.)

Troubleshoot by gaining access to R9 relay at P91 panel and check for 28VDC at terminal A2 when C1065 DC BUS 1
XFR C/B is opened.
28VDC should be present if R9 is operating correctly.
If no power is present verify 28VDC at A1 and X1.
If 28VDC is present at A1 and X1, then the problem is either a bad R9 or no ground signal for R9 from SPCU which
goes through M1474(Integrated system accy unit.)
Refer SSM/WDM 24-32-11.