I told him to speak to Marina, who is our contact from Ellis’s office for all of these problems. Marina called the landlady, who called the building maintenance, who then called Marina back, who then called me. (It would be so much simpler if I could just speak the language!)
The result of all these phone calls, and the cause of the “sound of running water” in our bathroom: the architects didn’t want to ruin the look of the building from the outside, so all the drainage pipes from the roof actually go through the building. Since the weather has warmed up, and the snow is melting, the water from the roof is draining down through the pipes inside the building, and this is what we are hearing.
Pictured: our bathroom drain.