HI!

I have a problem. We live in a 200 year old farm house with the old dirt cellar. Recently we had central heat and air put in the house. When the duct work was done an awful smell came from the holes in the floor. It smells like something is dead down there. It is coming from the holes in the floor where the vents are. When the guys doing the work were down there they did not say anything about something being dead down there. I have no idea how to get rid of it. HELP!!