One of my favorite things about the Star Trek franchise are all the great ambient sounds that represent the engine noise on the various ships. My favorite ambient noise from the whole series is the engine idling noise in TNG. I have cleaned up a sample from the actual show and then looped it for an hour and half. Really great for ambiance and imagining that you are in space.
Here is a download link for any interested in the original sound file for the video. Its saved as a .mp3: