A bill clattering crow

A few days ago I was walking past a wooded area in University when I heard a drumming noise. I though it was a woodpecker, and I stopped and watched. Not at all! A Carrion Crow sitting on a branch made this mute, bill clattering noise - in the most pure Stork style. The mobbing rattle is vocal, but similar in tempo. I managed to take a short video on it, must get round to upload it to YouTube and post it here. When clattering, the crow stood up and fanned its tail. It kept repeating the noise every few seconds. I could see no obvious context - no predators to be seen or even other crows.
More reading on Crow vocalisations here.