Suspected carpenter bees

  In my attic I found about six months ago like very fine dark sawdust in one place under a double rafter. The attic is foam insulated up to the sides of the rafters so only their central part is exposed. There are no holes or any visible sign of insects. I cleaned it away and placed a tray but nothing came again until last week, when the same dark particle dust fell on the floor on the same place. This time a cobweb hangs under the rafter so I can see that this dust comes from the very narrow joint between the double rafter. From the outside nothing can be seen as it is near the crest of the roof and anyway covered by asphalt shingles. Do you think carpenter bees are at work and which isthe best method to get at them as the only opening is that very narrow separation between the two combined rafters?