What is humming? Are you sure it's the G5 or is it the UPS? If it is the G5, make sure the voltage is set correctly on the UPS, it could be too much for the power supply.
I'm guessing it is your power manager that is humming. (High pitched, right?) My G5 does this when I transfer large files using multiple interfaces at the same time. (Hard drive, firewire, USB, ethernet) The noise is normal but has had many people worried. Me too for a while. I think there is a solution that temporarly fixes it in somethign called CHUD tools, but I am one of the least qualified people here to tinker with such things. Any advice I give you with CHUD will surely cause a melt-down.
Naa. I wouldn't sweat it. Most G5's sound like they have little sparks in them when cretin things happen and from what I can tell it is perfectly normal.
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That said, I know nothing about UPSs...good luck.
APC Back-UPS RS 800 Given to me by a friend who said it would work fine. I'm beginning to wonder....