My gas furnace which has been running for the past 4 months has started smelling just like it did when we first got it running again this fall. There is a musty, metallic odor that permeates throughout the house when the furnace kicks on and the heat starts blowing through the vents. Any ideas what could be causing this smell? It's not a gas smell. I also just cleaned the furnace filters about two weeks ago. I noticed this smell within the past few days.

Thanks for your input!

Is the smell constant every time the furnace kicks on? If so, you need to get a pro in there to check for carbon monoxide. Even though co is odorless, the byproduct of burnt gas does have a smell, and if that is what you are smelling, it may very well contain co which can be deadly. Don't take this too lightly. Have it checked right away. Good luck and please let us know what is found.