Hizzing / bad sound when running from battery power (Ups)?:

Why is my computer (Imac) emitting a hizzing sound when running from the UPS (battery)? I am testing a offline UPS type: POWERWARE Eaton Protection Station UPS 500VA 250W. My Imac is suppose to use max 200 watts of electrity (Imac 24 inch. intel 2.8 Ghz, 4 GB memory). When running from AC there is no sound. When testing (pulling out the cord on the UPS the Imac starts emitting a disturbing hizz / sound)???

I solved it my self. It was OK (no buzz and fuzz) when it was plugged in (when the electicity line was down once and I was running on the UPS battery).....

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