I heard it often early this year, but only on the top of the hive. I was curious to see what the expeienced folks said. It sounds a heck of a lot like what you hear from an exceited carpenter ant nest.
I have seen no obvious signs of wood chewing; but might have looked right at it and not recognized.
My first thought was that perhaps it was the bees adding propolis along the joints in the inner cover, and the thin cover acting as a resonator to make the little pops of sticky louder....just a thought.