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. :-)
"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.
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.
I used to have this problem off and on over the years with my Bryston and Conrad Johnson amp. My Jeff Rowland Design Group Continuum 500 has an incredible effect solution. The 500's Power Factor Correction takes ANY AC and converts it to 385V DC (yes, you read that right). The amp is dead-quiet in all conditions. There no transformer hum, 60-cycle or otherwise and no hiss. (You can put your ear to the tweeter at normal listen level with the source off and you hear nothing).

It take the regeneration concept, but doesn't convert back to AC, staying in DC for quiet operation.

Thanks for the responses guys. It sounds like I'm just going to have to live with it since any real solution would break my bank.
A brief addition. The Equitech website may be of help as it offers quite an overview of power problems. If not you might give them a call. I found an Equitech 2Q solved my hum problems. Their Balanced transformers are quite different than regular isolation transformers. Costwise you could try one of their "blowout" units, if they are available.
Halogens!!!! & other similar noise makers like dimmer switches & PCs! Halogens are deadly. They dump so much noise on AC lines you can hear the trafos across the room. Even if they're on another circuit.

You'll know instantly if you have this problem. Turn the lights off and listen.
Transformer hum is often caused by the 5th harmonic on the AC line. This harmonic (in the US, 300Hz) will also cause silicon rectifiers to radiate a lot more noise, and AC syncronous motors to develop an internal force that opposes their normal rotation.

This is a distortion on the line and can be exacerbated by power conditioners that have an internal isolation transformer, if that transformer is run above about 50% of its continuous capacity. The distortion on the AC line goes up by the current draw vs the source impedance- when the source impedance (which may be a transformer in the local power grid station) is low the distortion will be lower even with higher current drawn against it.

Those cylindrical transformers that hang on the power poles are a source of this distortion if there is too much current drawn from it or if the transformer is having problems.

There are two ways to filter the 5th harmonic. The most common way is by using a PI filter tuned to 300Hz, which involves some rather substantial chokes. The other method involves an isolation transformer and an error-correction amplifier that is able to correct any distortion from the output of the transformer, via a feedback winding. There is a company called Elgar that uses this technique, but Elgar does not target high end audio.

So the easiest way for most people to deal with this is communication with the utility. Sometimes they can be helpful... good luck!
"very frequently on Saturday afternoons"

I think that's your big clue. There's plenty of power tools and equipment that folks use on weekends to do various jobs that create mains interference beyond the scope of many conditioners to deal with.

It may pay to go for a walk around your immediate area when you notice the hum at it's worst and see if you can hear any obvious power tools or equipment nearby.
When you say that "all of the transformers" hummm . . . are you really sure? It only takes one for it to really, really sound that way sometimes.

The most common cause of hum and buzz in power transformers is loose core laminations in the transformer itself, or vibrating/loose metalwork around it -- and there's usually one particular component that's the major offender. If you do narrow it down to a certain piece, you might contact the manufacturer . . . they may be aware of a noisy production batch, or have a modification to the mounting to specifically deal with the problem.
Another issue with transformers humming is actually chassis resonance. It might be possible to isomerically mount the transformer and have it be quiet.