Weird smell coming from vents?

So yesterday when I woke there was this terrible smell in my house and its coming from the vents (heat), the problem is I have no idea what could be causing this weird smell so I don't know where to even begin fixing it. The smell is hard to describe, it almost smells like burning dust (when you turn your heat on high after not having it on for awhile) only much worse. I assumed it was mold either in the basement or under the kitchen sink making this smell but I just got done searching and couldn't find anything (I assumed mold because I have never smelled this smell before and don't know what could be making it). So as I went around trying to find out where the smell was from it smelt strongest when I put my face near the vents, but my furnace is in the basement and the smell is NOT in the basement.

If anyone has any idea what the problem I would really appreciate it. Thanks for your time.