Dear all,
Thanks a lot for your replies.
In the meantime I found the bug… it seems all HP Enterprise PCs (I'm using at home) make a strange (weak) noise as soon as ethernet cards are built in and transferring data.
For testing I used various 1 to 4 port ethernet cards (I210-T1, I350-T2, I350-T4 variants) as well as the onboard NIC (Intel 82579LM). Whatever combination was used, there was a similar behaviour.
So I did the same with a second HP Enterprise PC, same story.
My conclusion for the moment is, that it seems normal for those HP PCs as none failed yet and that my ears are sensitive to this kind of weak noise ... by the way, the noise was much weaker than a typical whining dammaged chinese transformator (which you can hear from far).
Well I really hope my hypothesis will hold for a loooong while ;)
Cheers chulio.