I'll go with the letter in the mailbox. Much better than asking them. In theory, it works. In reality, I've more often than not gotten shit from that. In my previous appartment, the guy upstairs was a noisy DIY-er too -- and with noisy I mean he did DIY stuff at 3am. I regularly went to ask him if he could keep the other people in the building in mind, but all I got in return was always that they had to keep him in mind. It became so bad that there was actually pee leaking from the bathroom ceiling. I went to talk to the maintainer of the building about it. He then went to talk to the guy about it. Their result? Although there were orange stains from the pee on my ceiling, they didn't see drips, so I had to shut up and keep that guy in mind. The next week, a couple of appartments on my floor were flooded due to some pipes that broke, but that was oh so unrelated.

And then I'll not mention when I did so when I was working abroad and some guys upstairs found it a good idea to have a party at 5am, playing that Barbara Streisand song at full volume while yelling through it. When I went to ask them if they could keep it quiet, they wanted to start a fight.

Or, at my dorm back when I was studying, I just got shot at with a BB gun or had ketchup and butter thrown at me (the guy living next to me had the same problem and the same results when trying to talk to them about it)

So yeah, excuse me for not going up there and talk to them about it.