I was picking up fan noise from my UPS at extremely high volume levels listening to low music passages and isolated the UPS by getting it off the floor.
I agree, as the previosu poster said, there is NO difference in sound, however, where I live we are subject to peaks and dips in the mains power. I use the UPS strictly to even out the line voltage. A good old SOLA 1500KVA computer UPS.
Jan