RPi power - extreme noise on audio

Hi there,
I am currently transforming my kitchen radio (http://cdn.pollin.de/article/xtrabig/X630548.1.JPG) into a raspbery-pi-web-radio. The radio had a pretty bad USB-functionality; the USB-board is connected to the electronics using a 6 pin connector: [Audio L, AGND, Audio R, Mute, DGND, 5V]

My approach has been, to use the 5V and DGND for powering the RPi, and the L/GND/R for the audio signal return (basically the same as the USB board, I don’t know what the Mute-pin is for though.

The RPI (A+) powers up without problems, the voltage is a pretty constant 5V.
Unfortunately there is an immense amount of noise as soon as the RPI powers up. It is a beeping and popping. There is a popping when the Pi’s LED light; when the Wifi-Stick LED lights, etc.
The noise is constant and not dependent on the volume setting.

Upon using a different power supply for the Pi (not using the internal 5V), the noise is totally gone.
Also tried powering a different Pi using the internal 5V and using the other one with an external USB power supply - the noise was gone as well. So the coupling of the audio and power cables seem to not be the problem here.
I tried to use a step-up power converter to boost a bit (to 5.2V) and to get a smoother power input - did not change anything.

Thanks for any ideas on how to solve the problem.
Would an audio hat (i.e. PiDACZero) help get rid of the problem?
I believe the problem is somewhat related to the DGND. I don’t know what the pin labeled “Mute” might be. Any ideas?

Thanks a lot!

DGND and MUTE seem to be the same (0 resistance); and I noticed that these don’t need to be connected - grounding on the Pi via the AGND on the 3.5mm Audio Jack.
Which is probably the problem…

Found the reason - probably a grounding loop problem. Ordered a Ground Loop Isolator and will try - optimistic that this will solve the problem. Stay tuned.