Came out of nowhere, seems to be concentrated in the upstairs and get worse when the furnace runs (which makes sense, the upstairs is stuffy, the downstairs is drafty, so fresh air comes in). Roommate suggested dead animal in the vents, the internet warns of a possible electrical malfunction. I have a carbon monoxide alarm in my room, so i'm clear on that end.
Anyone know what else it could be?
Monster Hunter: pissing me off since 2010.