Only during the heating season do I get an oder from a return air duct. In the summer, when the AC is used, there isn't an oder. The oder smells like something died in the return air duct. I smell this oder when the furnice is NOT running. The family says it smells like a gas leak but I've had the gas company to my hause, who have told me there is no leak. What could be causing this oder and how can I eliminate it?