Found it. Wasn't the monitor. Wasn't the ceiling fan. Wasn't in the floor or in the attic. It was in the closet.
Ready for it?
Care Bears Bedtime Bear's Caring Dreams. A book. One of those books with the buttons on one side, where you're supposed to play the sounds along with the story.
But a freakin' Care Bears book--one about good dreams, at that--was causing the most nightmare-inducing sounds I've ever heard in my own house.
I never liked the Care Bears.