noiselessly

äänettömästi In a quiet manner, without any noise.
1912: (w), (w), Ch.5:

Not thirty paces behind the two she crouched--Sabor, the huge lioness--lashing her tail. Cautiously she moved a great padded paw forward, noiselessly placing it before she lifted the next. Thus she advanced; her belly low, almost touching the surface of the ground--a great cat preparing to spring upon its prey.

The Bat—they called him the Bat. Like a bat he chose the night hours for his work of rapine; like a bat he struck and vanished, pouncingly, noiselessly; like a bat he never showed himself to the face of the day.