Hi, also have this issue.
To me, it just sounds like the phone's amplifier or DAC has a really poor SINAD. The reason you only hear the noise sometimes is because, by default, pulseaudio turns off audio devices when they are not in use in order to save power. If I'm playing music or something, the noise is always just there. But, if I'm not playing anything, it goes away. Also, like on my laptop that's running Linux, plugging and unplugging headphones causes the DAC/AMP to fire up, so that's why you hear noise with that.
More evidence supporting this is that the noise is just as apparent when using headphones as it is when using speakers. It is likely just a result of the phone's poor quality DAC/AMP.
To me, it just sounds like the phone's amplifier or DAC has a really poor SINAD. The reason you only hear the noise sometimes is because, by default, pulseaudio turns off audio devices when they are not in use in order to save power. If I'm playing music or something, the noise is always just there. But, if I'm not playing anything, it goes away. Also, like on my laptop that's running Linux, plugging and unplugging headphones causes the DAC/AMP to fire up, so that's why you hear noise with that.
More evidence supporting this is that the noise is just as apparent when using headphones as it is when using speakers. It is likely just a result of the phone's poor quality DAC/AMP.