This signal is first discovered when the Sole Survivor activates the relay dishes at radio tower 3SM-U81. The source of the signal can then be tracked to a ham radio inside the Greenbriar bunker by the Crane River.
Please, anyone - help us! We're stuck in an old bunker next to the river. Those things are crawling around up there... we're trapped! If you can hear this just... kill those things and get us out of here. Please, we're running out of food. I'll give you whatever you want.
This has been a pre-recorded message. Message repeats in three seconds.
The Greenbriar radio signal appears only in Fallout 4.