dirty air?

hey guys,
I hope you can help me. I recently traded my banger taco for one a little nicer. It's a 2000 taco 4wd 2.7 access cab. I love the truck. But there is one annoying problem that has me and my friends baffled. When i turn the air vents on, dust, dirt occasional debris of some sort, bellows out of the vents. The truck is very clean in appearance and would apear to have never been off road. But this debris thing is annoying. I dont know if it is safe or harmful to my 10 month old baby girl. I vacuumed the vents, did not help. I opened the vents to outside air and blew compressed air out, did not help. I have let it run with the ac on and off, still the same problem. It seems the bellow of dust and debris comes out when you change the setting on the right from floor to vents and etc, Have any of you had this problem before? I checked this model does not have a sort of cabin filters as some of the other manufacturers do. We even removed the ductwork under the glovebox and only saw fine scattered debris but we ran vacuum up as far as we could. My father in law mentioned maybe this thing has been underwater, KATRINA? oh well the truck is completely fine otherwise. I was hoping maybe to find where this connects to outside to clean it out or put a home ac filter cut down inline the ductwork somewhere Or could all this dirt be in the heater core?
With the dirt there seems to be debris from a gray or greenish color gasket or ductwork seal that flies out on occasion. If any of you know how to fix this problem we would be very thankful. I hope this can be done. I dont want to get rid of a perfectly good 4wd taco over something like this. But the wife not happy about this poof of dirt and debris in there with the baby. Oh and this is not the problem with foul smelling air, i have read about it before, my old paseo used to do that, trap water and stink. Thanks again, this is a great website for people that like to get a little mud on the tires.