General:
* Quietly
* Hush-hush
* Soundlessly
* Noiselessly
* In silence
* Without a sound
* Secretly (implies intentionality)
More specific:
* Discreetly: implies carefulness and avoiding attention
* Stealthily: implies movement with caution and secrecy
* Subtly: implies a subtle or gradual change, not a sudden silence
* Unnoticed: emphasizes the lack of observation
The best synonym will depend on the context of your sentence.