Static buzzing noise if I use a second monitor when Ubuntu is idle (Ubuntu 18)

I reinstalled Ubuntu 18 and I'm having a problem when I use a second monitor (I've tried both HDMI and VGA ports).

My problem is a static buzzing noise on the external speakers if I'm not playing any sound on Ubuntu 18 (the noise disappears if I'm listening to music or watching a video) so I don't thing it is a problem related to hardware (it worked fine on my last ubuntu installation and it works fine when I use windows).

I also found out that this sound disappears if I open the Sound settings...

What can I do?

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