Transformers don't make sound when they are shut off. Therefore, it is likely that maximum sound will result at maximum power transformation (at which time the magnetic core is saturated). For a given amount of power transformation sound can be transmitted to a different foundation that may resonate and produce more sound. Also if the transformer is partially enclosed it might be possible to chanel and direct the sound in a given direction. An iron object in contact with the transformer core may also emit sound, especially if tugged upon (similar to a growler).
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