One of the features that Apple added to the HomePod with the 13.2.1 update is the ability to play ambient sounds. Here's how to get them to play until you tell them to stop, or on a timer.
Once you get a handle of what Siri is looking for, the syntax is easy. You've got seven six options for ambient noise, above and beyond white noise.
For instance, to get the HomePod to play stream sounds until you want them to stop, say:
Hey Siri, play stream sounds
For the white noise generator, say:
Hey Siri, play white noise
To get those sounds to play for 30 minutes and shut off, start the sound of choice and say:
Hey Siri, set a sleep timer for 30 minutes
In our personal experience, we like the rain, stream, and fireplace sounds. Apple's stereo separation on a pair of HomePods is good —and it won't let you alter the volume up or down.
There is repetition after about 15 minutes, it seems, but you really have to pay attention to the loop to see it. If you're trying to get to sleep, or focusing on a task, you won't even notice the repeat point.