Does 'it' tap when you're standing right over it? When you lay your palm above the affected area can you feel the vibration/pulse? If not, is it possible that it's actually occurring somewhere else and the sound carries to a different spot?

The Death Watch Beetle sounds very much like what we call carpenter ants. They differ from termites primarily in their affinity for damp wood, I understand. The exterminator told me that southeast Alaska has an ideal climate for carpenter ants- cool, moist, lots of trees, lots of houses made of wood.

There are many horror stories of the effects of neglected carpenter ant infestations- houses that have turned to shells that implode and collapse in upon themselves. A building I know had one wall ruined by them. The exterminator treated the nearby areas for the critters, the construction contractor put in a new wall.