Noticed this the other day (I normally have my ROCK and Digione player with aluminum case inside a cupboard).
The noice is like a when a HDD is working very hard but maybe a bit more high pitched. It sound like this even when nothing is connected except the powersupply. I clearly hear it from my listening position when the door to the cupboard is open.
Is this normal ore is something maybe wrong with my RPi 3 Model B, or something else?