Got no clue what's up but I don't get this on my Gigabyte Odin 720W + Powercom Black Knight Pro 1000AP combination. My UPS does get quite loud though when powering my system through the battery.
I would suggest completely discharging your UPS though (so let it run till it shuts down from low battery).
I would suggest completely discharging your UPS though (so let it run till it shuts down from low battery).