The heating vents in our new home are in our floors. While recently cleaning them out I found about 1/2 inch of dirt in view in the front room vents. While vacuuming both lengths of the ducts as far as I could reach with the vacuum hose (3-4 feet) I could hear that a lot of dirt was being sucked up. Is there an explanation for this. I expected a pile of dirt under the opening and some dust in the ducts but not a pile of dirt in the ducts.