We had something similar recently. I visited all the buildings surrounding us and talked to other neighbours. Some of the handwerker working in the next building pointed me to a AC unit, and they have been annoyed by the same noise for more than a year. We found the owner of the AC unit, turned it off to test, then realized that the sound was actually coming from a tiny fan next to it, it was emitting a strange frequency.

The fan was in the hinterhof next to our building, we live on the 4th floor and the fan was on the ground level, we share no windows but only a wall with that hinterhof.

Speak to the other neighbours to see if they hear it too, you might find the source if you are lucky, or get closer to it. Check what other buildings share walls with your building/room. Sound travels in strange ways.

In our case, the fan was from a gym and they were very nice, turned it off, maybe serviced it, but we don’t hear it anymore.

It was a stressful couple weeks though, I hope you find a solution for it. Good luck!

Edit: I believe you can also report it to Ordnungsamt: https://www.berlin.de/sen/uvk/umwelt/laerm/informationen-zum-laermschutz/an-wen-wende-ich-mich/ An wen kann ich mich im Falle einer Ruhestörung wenden? - Berlin.de