The Hush

The Hush was a living sound originally created as a weapon of the Last Great Time War by the Time Lords. It would devour and steal the voice of anything that made a sound.

It was originally used in war, but since it was attracted by sound, it got into Radio Bravo because the transmitter was beaming its signal in the wrong direction. It had no physical appearance, according to the Tenth Doctor. The Doctor indicated it had been invented by the Time Lords. After it devoured the crew of Radio Bravo, the Doctor trapped it in an audio recording, but the Hush escaped when the recording was recovered. (AUDIO: Dead Air)