I have tried to ground them together but failed. The noise is still there, maybe I have grounded the wrong wire. From your diagram part C is different from mine, so I just find the only black cable and ground it to the same point as I did for CPU case. Could it be the grounding of the case is not good enough that is causing the problem. After that I used the isolator and the sound goes away.
When I increase the volume from the HU, seems like the volume level is unchanged. If I have input to the HU thru AUX, shouldn't the volume increase when I increase volume of HU? I have connected output of sound card to the female RCA of my HU. There is another stereo like mini jack, don't know what is it for and should I connect output to that input.
I can't find the manual for my HU. Model is HEIDELBERG CM127, only found HEIDELBERG CD