As someone else already stated, the likely culprit is DC offset. You can blame your power company. While an isolation transformer will stop the gear from humming, the same DC offset may cause the isolation transformer itself to hum.
Why do my transformers hum sometimes?
The power in my home theater seems to be especially bad. In order to improve things, I'm using a Monster AVS 2000 voltage stabilizer for my main components, an additional filtered power strip for the front end components and an isolation transformer for the subs. At times (very frequently on Saturday afternoons), all of the transformers in my system hum audibly. Anybody know why that is? My guess is that the 60 Hz AC sine wave becomes distorted somehow but I don't really know.