Why do my transformers hum sometimes?

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.