Why do my transformers hum sometimes?

"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.