This is a Toyota 4Runner, 2000 , 108K miles, V6. When the blower switch is turned on there is an audible restriction in the duct system. Sounds like a sheet of paper or something in the passageway in the dashboard. Higher the fan speed, the deeper the sound. Is there a cabin filter, could this be the cause or some way for debris to enter the ductwork?

Any help, much appreciated.