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.
You may be experiencing a high level of root and/or residual DC offset (DC leakage) in your incoming AC thanks to our utility companies. Many of us are actually experiencing some very high levels of DC signal in our AC. Some way in excess of +/< 12Vdc. However, unless it's detected audibly or in a mechanical manner (such as in your case), it generally goes unnoticed.

Occasionally, when a distortion signature is noted in an AC sine, it is often DC leakage. So you're correct in assuming a distortion in the AC signal.
Here's another wrinkle --

In my old house, I used to get humming transformers in my audio gear. I installed dedicated 20 amp outlets and the humming went away. So, it seems that sometimes transformers hum when they don't get enough juice. Dedicated circuits might also help.
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Because they don't know the words?

(Sorry, it must be Friday!)
Have a nice weekend. :-)