Just west-southwest of the tower, at the base of the drop-off, is a sewer grate to a drainage chamber. On entering, the door to the right leads to level dependent feral ghouls hovering around a ham radio, making them responsible for the broadcast. The ghoul sounds can still be heard in the signal even after the ghouls are killed, even if you turn the radio off ("click"), then back on (you have to select the station on your Pip-Boy again.)