If engines are firing or mechanical parts are moving they will likely cause vibrations that will travel through physical connections so you will hear them. But if nothing on the ship is moving or creating noise then there is no ambient sound and you would likely start to become very keenly aware of the noises your body makes. They build perfectly sound proof rooms here and apparently from people who have stayed inside you stsrt noticing your saliva squishing around, the dull thump of your heart beating, gurgles from your stomach and gut etc.