Transformers are made up of many metal laminations held together with bolts/screws. Over time these four bolts can become loose and vibrate. Open the case and tighten the four bolts snugly on the corners of the transformer. Also, the transformer generates a large magnetic field and can cause the metal enclosure to vibrate at a 60 Hz rate. Tighten all fastenings on the enclosure as well. Using your hands/fingers to hold/squeeze/apply pressure to the enclosure may dampen the vibration and will tell you if it is the enclosure that is buzzing.

Disconnect power before opening the case to tighten the transformer bolts!

If tightening the bolts doesn't solve the problem, then it is likely there is a problem with the stabilization circuitry. Note: a small amount of slightly audible hum is normal with most transformer powered devices.