I posted a message some months ago about this same issue. My not so ds of 3 1/2 years has a habit of singing himself to sleep, this has now gone on for over 2 years and I'm at the end of my tether.
At bedtime it takes up to 30 minutes before he will go to sleep and he always does it by singing. When he wakes up in the night he then does the same thing for up to 2 hours at the top of his voice. Last night was a mammoth session of almost three hours. During this time my dh and I went from ingoring it to timing him out to try to get him to shut up. As the time went on I found myself getting rough with him and I am really worried about this. We are all exhausted, and I'm sure it's affecting my relationship with him.
We went to a sleep clinic where the helpful health visitor told me it's a unique problem and she's not sure what she can do about it. I just want to be able to teach him how to get to sleep without waking the whole neighbourhood up