Buzzing transformer means one of a few things. Either the transformer is overloaded (does it buzz when nothing is plugged in?), the windings are loose and buzz, or there's DC on the line causing core saturation.
I have had this issue, and I developed a process to vacuum impregnate transformers with varnish, in my garage. Basically you have a container of varnish inside a vacuum chamber, suck all the air out (under a vacuum, air expands and bubbles out of the varnish) - then heat the transformer in an oven to 150 degrees, for an hour, submerge it into the varnish, and pull a vacuum. You can see the air bubble out as varnish takes its place. I like to let it sit like that for a while, several hours or more, and then put it in front of a heater, and let it bake to dry for a few more hours. This basically locks everything into place. If the transformer was not vacuum impregnated at the factory, you'll never get it totally quiet without doing it yourself. Once a transformer has been properly vacuum impregnated with varnish, everything is locked solid and there's no more hum or buzz unless it is overloaded badly. I used this process successfully with old tube amp transformers, and also, with a Philmore step up transformer I use to power European equipment. It went from sounding like I'm in a substation, to almost dead silent.
I don't like the idea of pouring bitumen into a container, because it's very likely to create more problems than it solves, and more or less just ruin everything. Bitumen can be corrosive, and I wouldn't touch this approach, in this day and age.
