The hum you hear is "50Hz" hum from the 'AC' (alternating voltage and current) that powers the transformer and coils.

The coils are having their applied energy reversed and reversed back 50 times each second.   Each cycle moves the coils winding and any transformer laminations (metal plates).  So the 'metal' is 'rocked' 50 times a second, and it's that vibration that you hear.

The hum is more prominent in EMs, as they have a heap of relays that are all AC powered.  Regarding NIB, I'd say get a better quality transformer.  The looser the winding, the louder the hum.  I think Toroidal transformers produce less hum.

...'AC' be a good name for a heavy metal rock band. It might sound better if you add 'DC' even though DC (direct current) doesn't 'rock' the 'metal'!


(Tech geek stuff: You should be able to get rid of EM flipper hum if you rectify the voltage from AC to DC - but this needs to be done properly as a fully rectified voltage will need to be stepped down in voltage to not overheat and burn out the coils.  An after market idea for someone who can be bothered making and selling proper 'flipper zero hum rectifiers'.)