I used a browser app and also a downloader program to actually download the audio files from Youtube (on the free plan). Then they don’t have ads.
Another way might be to download and install Audacity, the free audio program. It can edit audio but it can also create white, pink and brownian noise in many formats such as MP3, WAV and so on. I created 1 hour of white noise in about 4 seconds and the saved MP3 was 7mb on my computer.
Alexa will also play white noise but I think it only does that for a specific time. Because it streams things and does not generate it locally, imagine the overhead if lots of people used those devices to stream white noise all night. That explains why Amazon added a clock to later versions of the echos, I think to limit millions of people constantly asking what time it was.