It's a fact of life that an electronic transformer will make a buzzing noise. The 60Hz electrical current passing through it causes it to flex slightly and vibrate at that frequency, a phenomenon scientists call "magnetostriction." A metal equipment case containing a transformer will act as a sounding board, magnifying the buzz and making it worse. You can, however, remedy the problem by isolating the transformer with rubber washers. The rubber absorbs the vibrations and quiets the buzz.
Turn your equipment off and unplug it from the AC outlet. Give the equipment 30 minutes to cool.
Open the case with a screwdriver. Set the screws in a safe place so they won't get lost.
Locate the transformer. Underneath it, locate the screws that mount the transformer to the metal chassis. Count the screws.
Unscrew one screw and measure its thread diameter with the calipers. On a screw size chart, use the thread diameter to determine the corresponding standard screw size. Purchase one rubber washer for each transformer mounting screw using the screw size as the washer size.
Remove the remaining mounting screws from the transformer. Slip the screws back through the holes in the chassis. Place one rubber washer on each screw. Tighten the screws back onto the transformer. Use moderate force --- don't squeeze the washers excessively. Be careful to not touch any exposed wiring.
Close the equipment case. Replace the fastening screws and tighten with the screwdriver.
If the equipment is older, you may find it already has some type of soft washers on the transformer screws. These harden with time. Remove and replace them with new washers.
Exercise caution when removing the case from any electronic equipment. Avoid touching any exposed wiring, especially near the power supply. Even with the equipment turned off, hazardous voltages may be present.