Help wanted with noisy mains transformer

[Tractionist]

It's almost certainly flux moving the laminations [the windings/former are doubtless too heavy to produce the frequencies you describe - but applying pressure to these should confirm this either way]. Boater Sam's solution seems to be the most elegant and it should stabilise both the laminations and the windings/former too. Failing that mechanically isolate it from the chassis via a thin sheet of soft cardboard or rubber. Don't over-tighten the mounting bolts upon re-assembly use 'Loctite' instead on the threads, but do electrically strap these bolts to the chassis via pigs tails or whatever.

[GrimJosef]

The fact that the transformer stays cool suggests that it's not running close to its limits. But if the flux in the core was getting close to saturation then a bit of DC offset on your mains supply (typically caused by a large asymmetric load nearby) could push it over the edge and into humming/buzzing. This can be a particular problem with toroidal trannies.

There are various circuits for DC blockers on the internet and I keep one handy on the bench. If I get a piece of kit with a noisy transformer then I start by giving the blocker a quick try. My local DC offset tends to be worse in the evenings. I don't know the reason for this but I do believe that that's when the nearby supermarket switches most of its bakery ovens on.

Cheers,

GJ