Characters typically make a certain amount of noise, and thus alert opponents of their PRESENCE.
But thieves, as well as characters able to move quietly because of a magical device such as boots of elvenkind have a chance to be absolutely silent when moving. 
This chance to be absolutely silent is given as a %percentage%, +/- modifiers,
and the character must roll d100 to score less than or equal to the %percentage% chance he or she has to move without sound. 
Success indicates silent movement and an improved chance to surprise an opponent or slip past it.
 
 

Players Handbook

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AD&&D