Have you sniffed the 3 other room mates or their belongings, under their beds ect.?

Until you find the source, grab a couple tubs of an odor absorber like Natural Magic or Citrus Magic. Under $5. One would be enough for a room but when I use them after tenants move out I use extra, works faster. You can find them at Home Depot & Lowes near cleaning supplies, and some smaller hardware stores in cleaning or paint department. They really do work and even tho they have a fragrance it doesn't put that scent in the air.

I'd suggest one right next to the heat register for sure. (Have you unscrewed the grill over your register & cold air return and taken a look into the ducts?)

Have you tried FebrezeSPORT? http://on.fb.me/FebrezeSport Actually, did you even know there were different lines of Febreze? Yeah, I recently discovered that the SPORT line seems to work better than the original formula