As the phrase that has pervaded the last season-and-a-bit of Doctor Who recently became a bit clearer, I was reminded that I had filmed this video:
I filmed this backstage at the Colchester Mercury Theatre and because I was surprised by its volume. As the light went on and off and on again, it made an incredibly loud clicking noise.
However, my camera was not up to the job of recording the sound at all, so you'll just have to take my word for it. Therefore this is a video of the noisiest of 'Silence' signs, rendered silent by the apparatus used to film it.
Which almost seems profound.