Head unit / amplifier buzzing sound

[Zywixx]

Hello from New Zealand!

I’ve got a 2004 estima acr30 2,4 petrol engine, japaneese import with a stock touch screen head unit. There’s a buzzing sound coming from the Speakers no matter of the sound volume set. It dissapear when i turn audio off so i gues this is an amplifier grounding issue. Anybody had the same problem? I’m trying to figure out does it have an external amplifier or it’s just a head unit have a built-in one? I was looking under the seats and behind the glovebox but found only a black box under the passenger seat which is a tv tuner. Don’t really know how it should look like. Any ideas?