I do not understand, is the UPS or PSU making the sound?
-Under load it could just be a small high-rpm fan cooling it down.
Either way however, your UPS looks like its a regular switch over, not an online UPS which means its running off the wall (as far as I know, I could be wrong) when not on battery power. The only way it can be overloaded as far as I know is if its running off battery.
Moreover, under load, I doubt you are consuming 300watts of power with those specifications.