Plan Premium, Country Netherlands, Device iPhone 6 iOS 11.4 15F79, Spotify version 8.4.67.665
My Question or Issue
iPhone speaker makes a whizzing noise while Spotify is playing on another device. (put your ear to the iPhone speaker to hear it)
Summary
In this summary Iexplain what I have tried to analyse the problem.
Basic noise
1) When I start the Spotify app on my iPhone 6 and I play a song the sound comes from the speaker at the bottom of the device.
2) When I pause the music it stops, as expected.
3) When I select Marantz M-CR610, with the music on pause, the iPhone makes no sound, as expected (the Marantz is silent too).
4) When I start the music with Spotify the music sound comes from the Marantz, but the iPhone speaker makes a little noise, which I would not expect. The noise continues even when the iPhone screen is off/black. It is a whizzing noise, not loud, but apparently the speaker of the iPhone is active.
5) When I pause the music with the Spotify app the Marantz is silent immediately but the noise from the iPhone continues for about 8 seconds.
Alternative
6) When I shut down the Spotify app on the iPhone while playing on the Marantz the noise on the iPhone stops.
7) Now I cannot control the Marantz with the app anymore.
7a) I open the iPhone music control (scroll up from the bottom of the screen).
7b) The default player is iTunes, after pressing this menu iPhone control connects somehow with the active Marantz (and opens the Spotify app in the background?) and the iPhone device starts to produce noise again.
Workaround
😎 With the Spotify app off and the Marantz playing, when I open de iPhone control again (scroll up) and hold the air-play-button a menu comes up for device selection (iPhone is selected, see image with Novastar playing on the Marantz while iPhone is selected).

When I select Marantz the music stops. When I start the Play button again (in the iPhone control) the Marantz starts playing and the iPhone stops with the noise.
Conclusion
End result is some control over the start/stop of the Marantz music and no noise on the iPhone. Drawback of this situation is that start and stop with the iPhone sound control is slow and I cannot select new Spotify songs because the app is turned off.
I hope you can fix this iPhone noise and make my iPhone a silent remote control for Spotify.