lol I had this recently until I replaced my heater matrix and found out exactly where it all comes from!!! The control flaps in the heater box are all covered in foam, over time the adhesive deteriorates and it starts falling off. The only problems that it will cause are that eventually you wont be able to direct which vents the air is coming out of (it will come out of all of them) and you will always be getting a mix of hot/cold air.