It varies from day to day, but the sounds I consistently return to while writing are usually ambient(ish) in nature.
For the past few months, I've been coming back to the album wander into by ann annie (you can find it on bandcamp). It's got enough motion to keep me interested and engaged, and enough space to get lost with. ann annie is one of my favorite artists working with modular synthesizers. To my ears, modular synthesis often tends toward the loud and grating. While that's compelling and beautiful in many contexts, it's not what I gravitate towards for pleasure listening or especially for background music as I write. ann annie's music, on the other hand, has a textural subtlety that I really appreciate.
I'm also a huge fan of the website (and mobile app) myNoise. There are dozens of sonic environments to choose from and they're all customizable - you can layer as many sounds as you want, and even change the shape of each environment with EQ-like sliders. Mr. Rhodes and Autumn Walk have been my favorites lately.
Finally, if I want something a little more high-energy, I write to music that's on the mellower end of math rock - more complex and not overly obtrusive. I highly recommend the band Covet. Their music brings out distinct harmonic and emotional colors that help me "go deep" as I write, if that makes any sense.
P.S. In case anyone was wondering, I didn't remember the details of my dream enough to write about it at length. All I know is that it involved Kate McKinnon checking me out, which felt both odd and gratifying. :-P