Because I believe in transparency it's important to first note that the location of this apartment was absolute perfection. Hampstead Heath Park is so very close along with the tube, bodegas, restaurants, and shopping. Locationally this was a great spot.
However, it was the loudest apartment Airbnb I've ever stayed in. Upon arrival we discovered the upstairs apartment was under construction with zero forewarning. Additionally, not only was their contruction but when they weren't working, the children upstairs sounded like a pack of elephants, not limiting themselves to their home but also playing in the stairwell past 10p. Typically this wouldn't be such an issue, had we not had our 3 year old with us who had difficulty going to bed all 8 nights of our stay because of the noise (we even had sound machines.)
I understand Hosts don't have much control over their neighbors but definitely take this into consideration if you move forward with reserving.