οὐκ ἔστι σιγᾶν αἰσχρόν, ἀλλ' εἰκῆ λαλεῖν → keeping silence is not shameful; speaking at random is (Menander)
P. and V. ἡσυχῆ, ἡσύχως (rare P.), Ar. and V. ἀτρέμα, Ar. and P. ἀτρέμας, ἠρέμα (Plato).
silently: P. and V. σιγῇ, σιωπῇ, Ar. and V. σῖγα.
noiselessly: P. ἀψοφητί, V. ἀθορύβως.
without trouble: P. and V. ἀπραγμόνως.