You have moisture getting behind the walls. My guess is that has attracted a colony of carpenter ants. This is something you need to act on quickly as carpenter ants are destructive to the structural wood. The damaged area needs to be cut out as suggested by master of none. Removing the moisture source will be a big step, but you need to deal with the carpenter ants NOW.
Try the site at http://www.bugspray.com/article/carpenterants.html
The site gives you all the information you need, and suggests products for extermination and continuing control for carpenter ants.