it is normal for a power transformer to "give" sound while it transforms power-
let me list the typical sounds -
step down (cheap) - uuuuuuuuuuuuuuuuuhhhhhhhhhhhhhhhhhhhhhhnnnnnnnnmmmmmmmmmmmm
step down (expensive) - ehhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
servo - yehhhhhhhhhhhhhhhhhhhhhhhhhhh
cvt (constant voltage transformer) - trrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
step -up - uuhmmmmmmmmmmmmmmmmmmmmmmmmm
power regenerator (not a transformer) - .............................. (silence)