As far as I know, the cracking is a sign of danger or fright.
I feed them on my balcony. And for the first time I heard the sounds of close interaction, namely, as I understood, the hierarchy conflict. These are small and quiet squeaking, I can not describe them, but they are completely different sounds than they say constantly :)
Even more interesting, as soon as I first began to feed them, they flew to the balcony, and then could not find a way out. And now every bird knows how to fly to the street.

By the way, in Russian, these birds are called something like"sinitsa".
It was so called in the old days. It is the onomatopoeia of their screams when they call other birds. "seen-seen-seen!".
But even earlier, these birds were called "grasshopper". For their behavior, and especially for the way they deftly jump on branches.
And earlier, before that, they were called "zinziver". Again, onomatopoeia, from their spring songs :)