The only thing we know for certain is that there’s no obvious explanation for this 
@desperito - I can’t remember after all this time but have you tried disabling any unused wireless formats in your computer (e.g. bluetooth) ?
Nothing noted so far suggests anything other than RFI.
This type of problem is typically solved with a soldering iron, so it limits somewhat who can participate.
I disabled WiFi during the very first tests because I read sometimes it can interfere, I can’t remember if I disabled bluetooth too but I probably did.
Anyway since I tested it also with another PC and in another town, discovering I couldn’t hear the beep when testing in the other town I fear bluetooth or WiFi are not involved…
Can you please explain more of the answer? Are you talking about solder something inside the audio interface, or inside the PC? And what exactly?
Possibly, but does not necessarily have to be inside the interface.
From my July 22 post:
It would be extremely helpful to know the frequency of the interference, but high probability it is cell phone related (unless you have some other type of broadcast antenna near your apartment).
As a first attempt I would try Fair-Rite type 61 ferrite beads on the outside of the cable jacket, and in the connector a smaller type 61 bead over the center wire, and multiple small value surface mount ceramic capacitors in parallel between the center conductor and the connector shell.
Not guaranteed to work by any means, but would involve only a few dollars/Euros of supplies and maybe an hour of cable construction.
Hi there and thanks again for all the help, really appreciated 
Unfortunately I have basically no experience in cable construction… Could I ask you a more deep explaining on how to do that or - alternatively - to suggest me a tutorial I can watch to understand the idea and how to proceed?
P.S. I don’t know so much about antennas but I remembered the building across my street has this antenna on the roof:
As I said, I don’t know what kind of antenna it is and therefore if it could be involved in the issue…
… Does that ring any bells for you?
Hard to be sure from that angle, but looks like a 5G cellphone installation:
What part of the world do you live? I can try to find some references which do not refer to part vendors for example which are not in your geography.
Yes that is a cellular install at minimum, several sector antenna there. Plus a few others that I am not as such on, could be a microwave P2P for backhaul amongst several towers for instance.
Seablade
It would be really kind, thanks! I live in Italy
So (maybe) we found our bad guy?.. Could it be the cause of the interference?
Well anything is possible, the problem is the presence by itself doesn’t equate to the cause of it, so it could also be completely unrelated.
Seablade
Understood… And is there a way I can check if the interference depends on its presence?
Not without some very expensive RF analysis gear combined with a fair amount of specific knowledge. You would need to see if a broadcast change in cellular bands in your area corresponds to the presense of the beep. And to do so would require some pretty dang precise gear that starts at at least 4 digits USD and goes up from there. Actually it may start at 5 digits, can’t remember off hand as even I don’t own that gear.
Seablade
You partially did by going to a different city. You just did not conclusively show that it was definitely that item, it could still have been something else nearby.
But your experience matches similar anecdotes from other places of interference problems beginning when new 5G cell phone stations were installed.
Interesting.
I have a 5G radio on my desk here, although it has dummy loads and I really can’t get a signal unless I put my test phone pretty much on top of the radio.
I’m considering some proper antenna which would allow me to test some of this. Of course, “5G” is a whole raft of specifications, especially on the “new radio” side, so it very much depends on the specific spectrum, etc
Cheers,
Keith
Ok so basically I’ll never be sure since I’m not planning to own such a 5 digits USD gear ever 
By the way I’ve some little news: unfortunately I can tell you even the Radial J48 DI BOX doesn’t solve the issue… The beep is still there. At this point, I think we can say a DI BOX can’t solve it… Correct me if i’m wrong.
However, during the test I heard (in addition to the beep) other weird sounds like impulses and “sliding sounds” (like a glissando), which gave me the idea of the interference “going in and out of the signal”.
I managed to record them, but unfortunately I wasn’t on Linux and just could record them in MP3 putting my smartphone in front of the speakers.
Here is the link with the recording.
I can here some sort of “impulse” at 00:05, the “sliding sound (glissando)” at 00:18, the beep at 00:59 and two more consecutive beeps at 03:02 and 03:03.
I don’t know if this is relevant, but I’m sending it to you anyway. Just let me know if you need other tests
P.S. Don’t pay too much attention at the brushing and buzzing sounds, that’s me browsing the Radial J48 manual and typing on the keyboard trying not to get my muscles locked up while holding the recorder at the right height 
No difference if you engage the ground lift I assume? And this is with a short 1/4" cable going to XLR?
Haven’t had a chance ot listen to the recording yet, will try to do so in the next day or so.
Seablade
Here in the UK we’ve a government department called Ofcom which regulates all aspects of broadcasting, including phone masts. Is there anything similar in Italy?
Here in the UK we can request Ofcom to conduct a survey if a nearby phone mast is causing issues. Unfortunately it’s not free - but if any of your neighbours are also having issues, could you maybe share the cost?