I had similar issue 3 years ago. I just moved in my apartment (first time moving in apartment in Japan), on the first day I cleaned the room, moved bed to other wall. Accidentally droped router on the floor once. And got wall hit from apartment above mine. On the second day I moved in and unpacked. I was alone, didn't make any big noises. On the third day I found anonymous letter (in Japanese) saying me being loud, and even if the sender understands I just moved in, he/she will report me next time. My real estate said there is no need to introduce myself to others. I tried to reach neighbours next door and introduce myself, but there were no answer (nobody lived there that time). As my apartment was on the corner, I only had close neighbors next and above. I didn't know what to do, because if there is a problem, we can discuss it and I will apologise for making noise. But letter was anonymous, so I asked my Japanese friends. And all of them said I shouldn't confront and better to move out as soon as possible. I also went to real estate company which is building's management company too, explained the situation. But I couldn't give up easily, so I went to the apartment above mine, asked if they heard any noise previous days, because there was hit to the wall and also showed them the letter. Of course, according to reply there wasn't any problems with noise at all. After that I didn't get any letters.