Hi Reinhard,
Another one to remember is to have "load last session on power-up" NOT checked (it is in the set up window Ctrl +1) so you start every boot with a fresh default session and then you can load your saved session that you are working on.
If the settings in your session have this noise but when you start a default session it is not there then it is something within the settings in the session and I would go channel by channel to try and isolate it.
Other than that you could look at the gates and compression to see if any of those are in the chain. Turn those all off and see if you get this sound you speak of. There is not enough information to see what this might be I am just throwing things out there in hopes it will lead you to an answer.
Is the sound on every project, on every channel? Is the board making the sound when audio is playing, when idle? I know you said low level signals, is it every low level signal on every channel? I know it can be frustrating, right now I am working alone
and there is this annoying buzz in the room. I am trying to live mic, and I can't find the source of this awful sound. I'm like what the heck??? What is that???
Hope this helps,
Michael