Nothing. It's normal. It means you have to use the restroom or your food is digesting
When they don't feel like making noise and at night.
you cant. the sound is probably either a disk spinning, or the fan running so the machine doesnt melt. if its not making noise, its probably off or broken.
make sure your computer is unplugged and remove the fan from inside, and thoroughly clean it. this will "lessen" or remove the windy noise.
WD40 applied at appropriate places.
Lubricate.
The only reason your mac would make noise is if it is cleaning its self. If you first get a mac it might clean its-self more often. Once the mac is comfortable in its new home it will stop making the noise. I work at the apple store, and I can a sure you that your computer is fine. Remember to stay calm because all macs do make the sound, it is completely normal, and your mac will eventually stop making the noise, once it is use to it place.
making
Going to the garage to get new brakes before you gear box goes
Bleeding the air from them usually helps.
Unplug it.
You can buy 'Silent Spinner'; i know they are sold at petco. they make almost no noise and really are silent. the difference is that its plastic; it doesnt get rusty like metal wheels do.
either replace them or add motor honey to go for a longer perion without noise....