I have a old craftsman car batttery charger with 200 amp boost. Its transformer was always noisey from the day I bought it.

But it got only occasional use, so I didnt care....... it worked fine all these years..

well one day a neighbor borrowed it and forgot to bring it in doors. it sat out in the rain, for perhaps a week.

the hum went totally away. it appears the rain and snow caused the transformer to rust, tightening things up, ending the bad hum