I have went though and read a lot of the other answers and we have check out most of those things...

When the air comes on it smell nasty, I can not stand the smell and I do not want people to come to the house. We have a septic tank with a leech field, but you do not smell it when you are outside the house. It just started to get really bad when the heat started to be on more, when we first turned it on I did not smell it.

Any ideas, the vent to our house is in our master bedroom, and the septic is right outside our room but like I said before the tank is sealed and there is no smell outside the house. We are getting ready to have to put out house on the market and I would love to know where the smell is coming from so I can get a good nights sleep.

Thanks in advance for any responses :)

You have one of the more difficult situations to find and fix. Look for a vent or broken vent line on the in house plumbing to start. Look also at a floor drain near the furnace.

Is it a sewer smell? Could it be a dead animal somewhere,in the duck work ,inside the furnace,,