I'll start by saying my mom raised me to be very paranoid about smells (but she was the person who detected a fire in the wall of our apartment building when the fire was down the hall and she noticed before people who lived in the apartments with the burning walls).
We have the AC on and I notice the smell when its on (fans on). There are 3 types of vents in on our upper level - the vents with the filters (this is air going into the ac I think), small square vents that can be adjusted as to where they blow (cool air comes out of these) and then there are 2 medium sized vents - no filter, there is a grate, and I can see a large pipe going up from there. Sorry, I am so not techincal about these things. Anyway I notice the smell when standing under this 3rd type of vent. Its not a burning smell but not a rotten smell either (like a dead rodent). My husband said kind of like burnt toast (but he also agreed it doesn't smell like its burning). He stuck his head up in the attic and didn't smell the same scent (its not an easy attic to get into - we've never been up there). AC units are outside on the ground and heater is in the attic.
We've smelled it for at least 2 days.