I came across Ambient Mixer a while ago and let me tell you, this is a brilliant tool for all your ambient noise needs. There's a variety of mixes people made, all ready for you to download under a CC license. Or you can just play them online and change them to your liking, mute a channel, chance the randomness of a certain noise ect. If you don't find what you're looking for, make an account and mix your own. The site has a database of noises for you to choose from and a very easy interface to make your own mix with up to eight channels. You can also upload sound files from your own computer. There's a number of websites that offer free sounds, for example Free Sound.
It's a simple and very effective way of creating the right mood for your game, whether the characters are in a Tavern, on a ship or exploring a deserted spaceship.
And speaking of spaceships, if you are looking for some really creepy background noise for such a setting, look up 'NASA Sounds of Space' on Youtube. The Voyager probes recorded electromagnetic vibrations from the planets and their moons and it makes for a very eerie atmosphere, but it's not too loud or too intrusive. Here for example is Io: