To the OP-
I rented an upper flat from some friends and then, they built a house and eventually moved. The couple who moved into the first floor were nice enough, but he moved out and she started going to the local bars and bringing people back for parties after 2AM. She wouldn't turn the stereo on and increase the volume setting, she would crank it and THEN turn it on- familiar with Sylvester the Cat? That was about how I reacted. I let it go for awhile, then I called the Police. The first time, some guy came up and knocked on my door and yelled about it being her house, too and I let him leave- no way was I going to open the door for some random guy who may be coked up and drunk.
I ended up being very sleep-deprived and that's not something you want. I left years ago but still wake up at 3AM on occasion and there's no apparent reason for it.
Let the Police handle it, make sure the owner of the building knows
and tell them the reason you called. They can't legally retaliate, either. If the noisemakers are doing something illegal and the building owner knows it but does nothing about it, it may put them at risk of losing the property- check into that, too.
Above all else, make sure they won't do anything violent about you calling about this.