The Hush was a weapon built to destroy anything that made 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. The Doctor trapped it in an audio recording, but the Hush escaped when the recording was recovered. (AUDIO: Dead Air)