Loud bang from SMX-3000RM2U

[BillP]

This has happened several time over the last few weeks and I am not sure what to make of it.  There is a loud bang, like multiple relays cycling rapidly, coming from the unit.  Logging into the NMC shows not a single event that would indicate power transition or any attempt on the part of the UPS to compensate for voltages, etc.  There is no associated loss of power, even momentary, to any of the attached equipment.  I have set the UPS's sensitivity to a lower setting just as a test but I am not sure it will do anything.  Another UPS on the same AC circuit is not affected.

Any idea what this sound might be?

Thanks!

  The front panel LCD shows a message indicating the last transfer was due to "rate of change".  This begs the question, are there some events that the NMC is not capable of logging?