One of the most relaxing sounds I’ve ever heard occurs late at night in the spring and summer, usually after the frogs settle down. After wondering about it for twenty years I finally know who’s singing… Broad-Winged Tree Crickets! [Oecanthus latipennis]
Take a listen to this iPhone recording. They sing one continuous note for several seconds. Individuals enter and leave the chorus at random intervals and slightly different frequencies. This is apparently due to slight variations in temperature. The effect is very soothing!
This recording was made in the early morning hours after a rainstorm. About half way through you can hear a freight train whistle in the distance.