I am guessing mice. I don't think the fiberglass can kill them. You might try checking the outside around your roof line to see if you see any holes. I think a mouse only needs a hole the size of a dime. (yikes)

Could be racoons or squirrels. Look around the outside of the house for siding, soffit or roof vents that have been torn open. A can with a rag soaked in in ammonia has been known to drive them out. But you need to find the hole and seal it up so they can't get back in.

turns out to be the mother of all hornets nests, and unforunately I can't quite get the spray onto the net itself (too much blockage in the attic).

Ay-yi-yi! That explains the buzzing sound.

Good luck. Are you calling an exterminator?

i can spray near the nest, and hang some no-pest strips in the area, and around their access. it's certainly quieted down. I'm going to keep spraying and see if I can get them to leave.

I had the same problem. The wasps didn't damage the joists, but they did damage the sheetrock in the ceiling where they established their nest. I waited for cold weather, and they all died off. Then I cut out the damaged piece of sheetrock, cleaned out the abandoned nest, and made my repair. Just be patient. I decided I could wait rather than get a bunch of wasps mad at me in a confined attic space!

Had a similar experience with bees in the wall. The low buzzing or droning noise is the tip-off. They will die in the cold weather (only the queen survives) just as kudzu said.

I augered a couple of 1/4" holes in the ceilingboard with a screwdriver, shot in a good dose of Wasp & Hornet Killer spray, plugged the holes and waited an hour or so. After cleaning the residue, I caulked their entrance path and replaced some insulation.