That's funny- I noticed the same thing recently. I think they are coming from my sun visor. The foam inside feels a little crunchy and I noticed that whenever I pull the visor down I see dust particles falling in the sunlight. On mine the plastic cylinder that clips into the ceiling bracket is half broken and it appears the foam can escape from there. I'm thinking maybe I can plug the hole by sealing it up with a small amount of Goop type stuff.

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
http://home.4x4wire.com/erik