The ambient sound during the timer can be selected from 'Rain', 'Running Water', 'ASMR' which sounds like a burst of sound, brown noise , and exotic Middle Eastern music.

From SleepTimer to Super Single Sleep Timer, 3 apps to help you control the music and get a sound sleep ...

Struggle no more with restless nights! This sound machine offers 20 soothing sounds—like beach waves, crackling bonfires, or classic white noise—to whisk you away to dreamland. With 32 volume levels, ...

One of those features was background sounds, which Apple plans to ship in iOS 15. Background sounds are effectively built-in white noise features with additional accessibility options.