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.
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Because they don't know the words?

(Sorry, it must be Friday!)
Have a nice weekend. :-)
"Transformers can hum when line voltage is +/-10% of the transformer's voltage rating." -- Tvad

Might explain why it happens on Saturdays. More people home using appliances, drawing electricity.
thanks, guys. I wondered why my isolation transformers seemed to be humming more loudly since I moved from a dedicated-line house to an apartment with inferior power. There's no help for it though; they're needed more than ever where the power is bad. I've been trying to think of ways to deaden the sound. I understand that Shunyata's have a box-within-a-box design, with soundproofing between them, to address this, but a diy attempt at this would seem to create heat issues.
same problem as you and you learn to live with it...once I installed a regenerator to keep the voltage locked in at 120V this seemed to help greatly.