I've searched the forum, but I didn't find exactly the same problem or any similar solution to this.
I am using RPi4 with DAC+ ADC running Hifiberry OS the latest build.
Independet of the source I noticed well audible pop / click sounds under the following circumstances:
iTunes - everytime I hit play or jump to another timestamp while playing I, here this sound:
Spotify - I think it's the same problem, but not as audible as with iTunes (maybe because iTunes has this significant delay before playing):
Analog Input - Here it's a different noise when start playing:
RPi start up
1) plugging in power supply (noise not really an issue)
2) red LED on DAC+ ADC lights up (noise not an issue)
3) but than comes this cleary audible sine wave
I here no issues, when I play continuously. I tried several audio files and input hardware.