sounds like a defective unit; mine is absolutely silent. recommend warranty replacement.
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It might be defective but I had a Furman balanced 20A conditioner that had a large toroidal transformer and it will definitely buzz if you have DC on you AC line, most likely coming from a dimmer switch or other appliance that presents an unbalanced load to the AC line. The dimmer switch to the light on my stove hood would cause the Furman to buzz in the basement. If you are in an apartment or condo it could even be from another unit in the building. This is not an issue isolated to Furmans, it is a characteristic of toroidal transformers.