2006 Avalon xls 70k miles has just started throwing out a few pieces of what appears to be little triangles of felt like material through the vent when AC is on. Not a lot but is something breaking down?
A likely cause is a rodent nesting in the ventilation system. They may be chewing up carpet or sound deadening material. It may also be the foam that seals the ductwork sections breaking down. The material may be breaking down due to age or heat if you live in a warm climate. When the fan is turned on the air movement is blowing the pieces out the vents.
Ducting flap seals breaking up. see 'mikes' answer.