There was a video recently where YouTuber diodegonewild was fixing old power supply, he turned it on, listened to it and said "the transformer has DC offset on it, because it's humming at 50Hz, the magnetostriction doesn't care about polarity, healthy transformer hums at 100Hz, I think one of the diodes in bridge rectifier is bad" and he was right.

https://www.youtube.com/watch?v=bWxS2njU-JI&t=355s