Strange behaviour from a kookaburra

Hi

We have several nesting boxes in our garden, one of which is a kookaburra nesting box. This year a Kookaburra keeps flying in and seems to be trying to dig a hole in it. We can hear (even the neighbours can hear it) a very loud and continual tapping in there. It goes on for a long time and then the kookaburra leaves. They have not moved back in. 

About two years ago the adult kookaburra brought a baby snake back to the nest. Normally when they do this the snake is either dead or close to, but this one was very much alive. There was a battle royal between the snake and a baby kookaburra. We could see it on the edge of the entrance with the snake in its mouth but it was fighting hard and wrapping itself around the neck of the kookaburra. We were afraid it would fall from the box as it was teetering on the edge. Eventually the snake won and was dropped to the ground. However a day later we found the baby on the floor dead so it must have been bitten. Since then the kookaburras have not moved into the box. We recently cleaned it out, but there wasn't much in there at all. (unlike the parrot box which a possum has moved into). We don't know what the kookaburra is doing with all the tapping. Is this normal behaviour. It is not something we have seen before.

Thanks