Echoes were beings of living sound. They travelled the universe via radiowaves, absorbing sonic energy from the planets they passed.
History
A group of Echoes arrived on Earth, in the sky of New York City. They were being hunted by the Shreekers and the resulting sonic chaos drove some people mad. The Tenth Doctor along with Gabby Gonzalez saved the Echoes by destroying the Shreeker battleship. The Echoes thanked them as they flew away. (COMIC: Echo)