why do transformers make a humming sound??

Answer:

Iron or ferrite core transformers if operate at frequency between 20- 20,000 Hz of the applied ac voltage,
the core material's length is extended & contracted in length by very small amount at that frequency known as magnetostriction process.
Magnetostriction causes air being pushed back & forth at that frequency causing sound generation which is hummung sound.
Transformers without core do not generate that sound