@JimmyT said:
A knock on the door (or a note under it) and a very polite inquiry along the lines of “I keep hearing strange noises at these times – do you have any idea what it might be?” could solve a lot of problems before you start involving the cops or the council.
I had a similar problem recently with an upstairs neighbor (a divorcee mum with a couple of uni aged kids). Sounded like they had some gym gear with things falling/dropping on the floor at regular intervals. I just mentioned casually to her when I passed her in the car park one day that I could hear these noises in the building, but not sure what they were or where they were coming from (sounds like this can actually travel in strange ways in buildings). She said she hadn’t heard them, didn’t know where they could be coming from, but I haven’t heard them since.
As for the “brothel” suggestions, I understand that in NSW one girl working from home does not constitute a brothel and is quite legal; two girls in the same premises is a different situation.