Ambience is the KING of noise machines. I am surprised —- rather shocked, really —- that it did not make the list.

Ambiance (http://ambianceapp.com) is worth a look. It has 3,500 sounds, mixable, with alarms and playlists, and no data usage after you download a sound. I've been using it for years and have never found anything comparable. (And no, I'm not connected to the app or its developer in any way.)