With the phone hooked up to the amplifier, I heard the noise while turning on/off airplane mode.
Then I had the airplane mode off and turned off/on: wifi, bluetooth and 4G separate the noise did not appear (I was trying to pinpoint exactly what was making the noise). The noise only appeared when I turned airplane mode off/on.
With my laptop hooked up (and streaming via wifi) there where no noise, so it seems the phone needs to be hooked up to the amp for the noise to be heard.
I don´t know if it matters, but my phone is a Samsung s10e from 2019.