MyNoise.net is awesome. They have various shades of White noise and pretty much any other noise you can imagine wanting to listen to, as well as a few you’d never want to listen to. 
Can access via the web and/or via their iOS & Android apps.
Here’s an example. I’ve found that I’m usually very productive writing while flying–something about the background noise of a plane helps me get in the zone. MyNoise.net has a few plane noise generators, and I’ve discovered that I really enjoy working with my headphones running a B-17 bomber simulator:
Best,
Jim