https://youtu.be/lUt8rE9tYo0?t=21Here's a really loud transformer. Simple matter of the laminated plates loosening and vibrating when powered. "Generally" the hum or buzzing not an issue other than annoying. Which on yours, once on, heats up and expands enough to quite down.
Sometimes easily fixed. Often not & can be made worse. Sometimes plates are bolted which slight tightening may fix. Often welded but if it has corner holes, bolts can be added (as per video). If its riveted not worth the effort. If your converter has large transformer it my be better time spent replacing and installing a newer converter. Again don't even look at it without turning off ALL power,
If its something your unfamiliar with, especially older units, exposed internal parts can retain quite large residual power, enough in some case to kill-just sayin.