Portal/Transmission Received

The newest achievement, added in March 1, 2010 to Steam only, requires the player to find one (or more in some of the larger stages) radio(s) in each level and bring each to a specific point. A radio will have a red light on the front if it hasn't been brought to the correct location. The music will change to static as you approach the correct location, and then to a hidden signal when you have arrived, at which point the light will turn green. The achievement for an individual radio persists across saved games, though the light will change back to red. Since this achievement is only available after you have played through the game once, during the first play through only the initial radio exists and its blue light and signal will not change if brought to its achievement location.
 * Availability: Steam only; must complete the entire game first and play each chamber again. Completing a chamber or chapter does not enable the radios to be available. Game completion is saved on a per-installation basis - it is not saved to the Steam account, even if the Heartbreaker achievement is acquired.

The hidden signal that is picked up after bringing a radio to its achievement location may be a Morse code signal, or sounds similar to those found on modems. To get full benefit out of them, you need to use sound recording software, or otherwise extract the audio files from the game.
 * Hidden signal