Another one is https://mynoise.net/, which has a neat mode where the sliders will very slowly move on their own, so you get slowly adjusting soundscapes.
I don't know if it works offline though, and probably not when your phone is locked. They also published recordings on spotify though; some are an hour long, others you can probably download them and loop them though. https://open.spotify.com/artist/1gRJBUyCeihBrgcCtDdEfv?si=yL...
If you’re using iOS, there’s a built-in system for Background Sounds[0] that has quite a few options, and is built into the OS, which means it definitely still works while your phone is locked, or even otherwise engaged. It even gets out of the way for other audio and comes back very gradually.
This website quite possibly saved my sanity when renting a tiny one bed flat in London with my partner and newborn baby. The traffic noise was unbearable, it was the hottest summer on record and he was a terrible sleeper.
Also, if you donate to mynoise you get some download credits that you can use to get an mp3 of your preferred slider settings. Great option for using it offline and it helps keep the lights on.
I don't know if it works offline though, and probably not when your phone is locked. They also published recordings on spotify though; some are an hour long, others you can probably download them and loop them though. https://open.spotify.com/artist/1gRJBUyCeihBrgcCtDdEfv?si=yL...