To have drywall nails back out slightly is not unusual. This happens when the humidity increases and the wood swells slightly. This can push a nail out slightly. When wood dries out it shrinks leaving the nail protruding. When humidity increases again the same thing happen again. After a while you have a nail pop. This occurs more often when the nail has penetrated the edge of the stud. Just drive them back in and patch before painting.
As far as the noise you hear, the way you describe it, if it sounds like wood breaking, it probably is. Go back to the attic, pull back insulation and look for cracked or split ceiling joist. Look at roof rafters. Houses are built such that horizontal wood members rest on another piece of wood. Not just nailed to it.
Look at ends of joist and rafters, that would be over toward the eves and won't be easy.
I can think of no reason for a piece of wood to just break unless you have something really, really really heavy in the attic. If you just put something up there that you shouldn't, you might bow a joist but not break it.
Give a good look around, paying particular attention to the ends of wood members. Look the entire attic. Sound can be very tricky as to where it comes from. Let us know about the possibility of marine clay.