Nature:
* A hush falling over the forest.
* The stillness of a snow-covered field.
* The quietude of a deep ocean.
* The serenity of a sleeping volcano.
Objects:
* A tomb of silence.
* A blank canvas of soundlessness.
* A shadow cast by noise.
* A vacuum of sound.
Actions/Feelings:
* A heart beating in the dead of night.
* A mind lost in thought, a world away.
* A soul shrouded in secrecy.
* A gaze fixed on the horizon, unblinking and silent.
The best metaphor will depend on the specific context and the effect you want to create.