How old is it? I've seen a lot of transformers get loud as the resin hardens up over time. It's not a danger or anything, just the nature of the beast, considering that transformer is essentially a giant 75,000 Watts speaker coil, and if it gets a little room to vibrate, it will. It's a wonder they don't all hum as loud as a jet engine.
It's it's too disruptive, replacing it is about all you can do.
