A solution can be to decouple the unit from its mounting surface - perhaps using rubber grommets under it. Even better is to mount it directly on a stone or concrete wall - or perhaps a heavy wooden beam, or pillar if possible. That should absorb most of the hum.
Electronic transformers can have the same problem, but often at a much higher pitch. Here, you might try encasing it somehow - just make sure you can easily access it when you need to (click on the title to read more...)
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