What are some soothing sounds that have helped you relax and fall asleep?

For me when I was younger, I used to love the sound our furnace made. When the sound came through the heater vent in my room I instantly felt comfort and relaxation and it helped me fall asleep at night.

I also loved the sound of an airplane flying over high in the sky. I liked the sound of a helicopter that was not too low to the ground as well.

I am really interested in making a CD with a bunch of soothing, yet unusual soothing sounds like the ones I have listed. I need your help in doing so.

What are some sounds that are soothing for you now or were in the past? Please provide why it is/was soothing for you as well. Thanks!