A letter to a neighbour about noise should be as specific (and polite) as possible. It should include details about how the noise is affecting your ability to enjoy your time at home. It should also provide specific examples of noise and disturbances, when these occur and what you would like your neighbour to do to resolve the issue.
I am writing to let you know that I feel unable to enjoy my time at home due to the noise that you are making in your own home.
For example, I often hear you play music loudly after 11 PM.
Can you please make sure not to play music or make any excessive noise after this time?