I obviously don't know enough of laptops I must admitt. After reading Mac's advice I started to doubt about what I put in. So I started reading. Yes what Mac says is at least partly true, although I have read as well that it is as he also says a probability, so not a 100% guaranteed thing. The things I had been reading was also about interference but not as I said from outside sources, but more likely from the laptop's internal circuit. This was said to be the most likely one in some cases. To know if you got a groundloop, you need to listen to the hum itself. If it is a constant hum, not changing in "tone" (50Hz(like Europe)-60Hz(like USA) or harmonics of this tone (mostly 100Hz-Eur or 120Hz-USA)so depending on where you live) then it would be a groundloop, if the sound changes ( it would be interference). This part my studies at least still covered, so I don't doubt about that conclusion.

The most likely cause I had been reading: DC offset
The thing I did not know, which was mentioned and seems very acceptable in it's explanation, comes down to my lack of knowledge on laptops and their powersupply trouble. I write the simplified version of it. The power supply of laptops deliver noisy DC (causing interference), where the batteries deliver very pure DC(causing no trouble). The noisy DC from the powersupply often leaks out onto analog ports, and causes buzzing noises.

So if this is the case there would be another solution necessary. The solutions I have read are far too complicated for me to make it understood, even though I thought I did understand. I won't burn my fingers on that, but maybe Mac, who seems to have some electrotechnical background here has something to say about this and how to solve it. That is, if it really is an interference type of hum. If just a groundloop, I believe his ways, since he seems to be very sure about it and for sure I am not (with laptops anyway). Besides, I am pretty much autodidact in many ways, so my solutions in this matter probably do not cover the "safety-standards" for use with a laptop, or maybe anything at all. I am not a electronics but an audio engineer, so well who am I to bring up the good solution...

I'll be back...