From YouTube, a playlist of 12- and 24-hour-long videos of ambient space noise, mostly of the sounds of spaceships like the Tardis, the USS Enterprise, and the Nostromo (from Alien). I think the Death Star is my favorite:

Or the completely unrelaxing 12 hours of Star Trek red alert sound:

Sadly, the list is missing my favorite spaceship sound, Sebulba’s podracer from Phantom Menace. See also Super Mario Bros Sound Loops and Extended Star Wars Sounds. (via @finn)