I had same issues with an old Asus notebook. It is switching power supply of notebook pushing harmonics into audio. Workaround is to isolate ground rod of power supply but this is against any safety procedure in Europe. I lived with that for years. I think it should be more suitable to give a chance to modern universal power supplies for notebooks. But you should try before buying of course.
After that notebook experience I had another Asus and a Sony Vaio with no issues, such as my new MacBookPro.
In my experience this was an issue with notebooks older than 12 years, but maybe it’s coming back…
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Thanks for the explanation.
I wasn’t dormant in the meantime. I bought a Furman power conditioner (Furman AC-210 A E), a two-pole-PSU for the computer instead of the standard “Schuko” plug one, which is actually inside the safety regulation requisites (I think they just unite ground an neutral wires), a powered USB hub of higher than usual quality and - a bit on the tinfoil hat side - a handful of ferrite clip-on doodahs.
Since I’m literally drowning in work it will take a few days until I can carry all the gear to the basement. I will make a “documentation” for my personal use and take this step by step, from “factory settings” to full throttle with all those gadgets in use.
If any of these help, I will gladly post any findings/flukes.
(I’m pretty sure, though, that it’s the damn fact that the MOTU M4 is powered by the same USB wire that’s responsible for data transfer. Worst case I send it back and get meself an audio interface with external PSU.)
If you take power for audio board from the same outlet of computer supply, ground loop will not break anyway. This is why insulating ground of computer supply solves the issue, this way you break the loop.
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After too many work-filled days I can finally test my complete setup tonight (I have a “dress rehearsal” tomorrow. Will do this step by step, as mentioned before. Can’t wait to see when the loop breaks 
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Yeah well, took longer than anticipated. But I did this yesterday.
To cut it short: Naturally you were right, but I HAD TO experience this myself.
- Powering the computer with the 2-prong-PSU put the noise down considerably, but not completely, still high-pitched whine and other typical laptop dirt
- Isolating the output via the Behringer transformer box cut the noise completely plus quite a considerable amount of treble, so it sounds a bit like through a blanket. Must buy the same gadget but with high quality transformers - OR I build me one, but with the current price of Lundahls I think I’ll go the pre-assembled way.
Everything else, while not unnecessary, did not really help. Will keep the gadgets, though. The Furman because of safety for my equipment if there’s catastrophic power failure or something, the 2-prong-PSU as a backup if the original one should ever fail.
Have ordered an “iFi iDefender” (60 Euros) which allegedly can eliminate possible ground loops between audio interface and computer. I will try that one, too. It needs a seperate wall wart (i. e. a mobile phone charger) to keep data and power separated.Well, I’ll see soon, won’t I?