My Wall/Ceiling keeps on making a Crunching/Clicking sound- what is it?

My bedroom is on the top floor of a converted bungalow, so my ceiling is sloped on either sides. The noise is happening in the sloped part of the sloped ceiling, and I don't think it's mice, as I don't think they live under roof tiles... I was thinking may be some sort of beetle? or wasps etc.? enlighten me... (and its not pipes)