As he stroked her stomach, there was an almost inaudible sound.

A nearly inaudible sound like a footfall came from behind me.

He made a faint croaking sound, inaudible in the wind.

He felt inaudible sounds form and die in his throat.

All the others were tense, listening for inaudible sounds.

Again he searched his mind, closing his eyes like a man listening for an almost inaudible sound.

The girl on the bed made a slight, almost inaudible sound.

Ordinary, almost inaudible sounds needled into him, full of substance.

They heard inaudible sounds and saw invisible movement in the shadows.

Let's start with the "active" systems, those that send out blasts of invisible light or inaudible sound.