I had some garbage to bring to the incinerator, so went out to check. It's a good thing I was wearing slippers because the carpet was wet and squishy. The hum was from a suction machine to remove water. The machine was halfway down the hall, and its hose led into an apartment.
Apparently, there was a flood. It stopped raining, but it's very windy today (not that I've gone out for several days).
In April of 2002, a handyman knocked on my door one rainy night. My neighbor downstairs was complaining about water dripping from her ceiling. We found to my dismay that the area near my terrace door had turned into a large puddle. The water was not coming through the door from outside, but UP through the floor. In fact, much of the living room floor was damp under the rug. Luckily the electronic equipment is further in, and there was no obvious damage to anything.
It turns out there is a terrace drainage area under my living room, and this got clogged and backed up. I could see that the water on my own terrace showed no signs of draining. I spread towels around the wet area inside. Talk about sunken living rooms!
The next day, some guys worked on my terrace with a suction machine. Probably the same machine I just saw. It worked. Although I still keep a small towel on the floor inside the terrace door "just in case," I've had no new floods. I never did get my floor fixed or replaced; the wood is ugly and warped looking where it was wet. One of the many "things to do."
So my neighbor down the hall may have a similar terrace drainage area under her floor which flooded. You'd think being inside means you're dry, but not always. Scary.