Two story, midwestern suburban colonial built in 1947. Gable roof. Plaster walls and ceilings. House faces north, and ceiling joists run north-south direction. I am hearing rodents inside of interior bedroom walls and then running across my ceiling in a diagonal direction, i.e., northwest to southeast. I didn't think there would be any space between the ceiling and the joists that would allow free range movement like that. Am I misunderstanding how it's constructed, or is this a sign of a problem (other than rodents)?