It is never completely silent inside a space suit; you can always hear the gentle hiss of oxygen, the faint whir of fans and motors, the susurration of your own breathing—even, if you listen carefully enough, the rhythmic thump that is the pounding of your heart. These sounds reverberate through the suit, unable to escape into the surrounding void; they are the unnoticed background of life in space, for you are aware of them only when they change.
The Haunted Space Suit
by Arthur C. Clarke
The guy was writing brilliant prose while I was still in nappies.