Microlab Solo 9c Tweeter noise

[Audiomachine]

Dear audio enthusiasts,

I am having since a long time a noise coming out from my Tweeter speaker of a model Microlab Solo 9C.

I would describe it as a low wind, but it's hearable from a 3m distance, when the speakers are just powered on. The noise is coming from the main "right" unit, where the mothear board is, even when the left speaker is unplugged with audio input.

I have tested it with different power outlets 230V/50Hz.

The noise is not present after turning on the speakers. It starts to be audible after a few minutes. I assume, it has something to do with the transformer as it heats up?

Could someone of you identify what could be the cause and how to possible get rid of it?

It really bothers my when I don't listen to music and I have to turn off the speakers all the time.

Sample recording:
https://voca.ro/1iFQM1x8LmYK

Thank you for any kind of constructive feedback.