Keep the engines running

I remember the first time I watched Star Trek: the Next Generation on a television with a halfway decent sound system. Until that moment, I'd never known that every scene aboard the ship had been Foley edited to include a subtle underlying audio track: the constant thrumming of the ship's engines. 

Now you can listen to the Enterprise's engines in your own sleeping quarters, idling uninterupted for a full twelve hours.

If's that's not your ship of choice, you can fall asleep to the gentle engines of Deep Space Nine, the Nostromo, and the Millennium Falcon.