Let’s Kill Hitler Hidden Clue?
The Silence were mentioned in Let’s Kill Hitler, but didn’t actually appear. Or did they?
In the scene shortly after Mels’ regeneration into River Song, Rory mentions he’s getting a “banging in his head,” leading to a funny joke. But if you listen very carefully as he says this, a familiar sounding audio cue might suggest the Silence were actually present. The scene occurs around 19 minutes in.
Watch for yourself and make your own mind up.
Thanks to Jamie for pointing this out.