I haven't seen it first, but felt the vibration and after touching the window for about 10seconds I was sure that this wasn't my neighbours music or something else. It was weird because I didn't hear such deep sound before, it wasn't very loud, but had a very low frequency, I guess that's why the window vibrated so strong. One could feel the pressure or tingling on the skin.
Then it came into eyesight, I don't know if it switched that red triangle shaped thing on, or if I saw it after starting to look for something that could make such strong vibrations. Everybody would look for a car parking there with loud music, so did I. But I didn't see it, so I looked up. The thing I focused my eyes on first was to see, if there is some movement below it, like rockets, lights or anything, I was really afraid of what would happen next. I couldn't run away, it was just there. After starting to realize the situation that I was in, I hid at the side of that small room, but could still see how it stood there in the air. After some time, it flew higher and increased it's speed, without the red triangle shaped thing in sight, it was hard to make out. The first thing I did, was to call mum and ask, if she's ok.
Summing it up: It had some kind of engine that allowed it to stand still in the air and move to different directions, it was gray or dark, not sure. The engine wasn't loud enough to get aware of it from a distance, but when "near" you could feel very strong vibrations. It could accelerate pretty fast. It had no flashing lights, but it looked like there was something moving below it, I couldn''t make out what it was. I don't remember the shape good enough, sorry. A prominent thing was the large triangle shaped red thing on the front of it. Could it be infrared? Isn't infrared invisible to the human eye and why was that so prominently visible then?
Thanks for your account, these things will show up again if they are actively used. There's no question that such an aircraft could be built, I've been thinking about it myself. (Strong aviation interest).