The Hush
From Tardis Wiki, the free Doctor Who reference
The Hush was a living sound implied to be 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. (AUDIO: "Dead Air")
It was originally used in war, but since it was attracted by sound, it got into Radio Bravo, as the transmitter was beaming the signal in the wrong direction. After devouring the crew of Radio Bravo, the Tenth Doctor trapped it in an audio recording, but it escaped when the recording was recovered. (AUDIO: "Dead Air")
The Hush was not originally sentient, however it became sentient after being used as a weapon for some time. It has no physical appearance. (AUDIO: "Dead Air")
