Diry Air Ducts

I need some help here. Ive looked all over my W9 tryin to figure out where the gravel dust is coming in. I have a 08 and so everything has a pretty tight seal yet and I have the fresh cab filter on the outside but it seems like Im getting dust from somewhere yet. And everytime I turn the fan motor on I get a dust bath which I know once its in there its really hard to get out. Any help would be great!! Thanks.

 

All I run it on is the "recirc". Thats why Im really confused on where its coming from. Do you guys know would there be a filter under the hood on the fire wall at all? This is my first KW so Im still "learning" you might say. And do they really have vent cleaners they sell at truck stops and if so where?? Thanks for the input.