There are various apps that purport to generate white noise, but the free ones I’ve tried have all been strange or skeevy, and YouTube’s a minefield of allcaps woo and bad ads. Turns out that the best place to get it is a website: NoiseMachines.
The white noise generator is perfect, with an equalizer, presets and options to save and share your own. (Here’s one I made for you that I’m calling “24 hours of bridge noise but something’s very wrong with the spaceship“)
But there’s also lots of sample-based blends of noise, too, such as thunder, vinyl scratches, generative piano music (my setting), data centers, binaural beats, etc.
The creator is Stephane Pigeon, a sound engineer with various other similar projects to enjoy.