Hello,
my setup is a raspberry 3 with a suptronics x400 DAC.
In the sleeping room I heard a noise - finally I figured out that it comes from the speakers.
Should volumio mute the speakers when no playlist is played 
Or other question - do you have this troubles also with other dacs?
I have a hardware tuner for presetting the volume. and when it is set to 50% I clearly can hear this noise in standby mode,…
GreeT`s eRich
I think that it is a board issue. As your board does power conversion, D/A conversion and amplification, I guess somewhere it picks up some static noise that get amplified even when the Pi is not providing an I2S signal.
Easiest way to test I think is to turn on your Pi without the SD card so it doesn’t boot anything. If than you already hear the noise, it is not the Pi.
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I agree with PatrickB. I own this board, and I would add that it’s sensitive to the electrical environment. Power cable near the board may cause this.
Take care the power supply cable does not touch the board.