Oh my, bad smells have got to be the worse thing ever. Just a couple of suggestions from far away from where ever you are, I know because I just check the thermometer and it indicates 2 degrees above zerro and everything is white with snow outside!
Follow your nose. Open all compartments and smell to see if it could be in one of them. Bad smells could be:
a. Rotting food (maybe something rolled under the couch or somewhere else)
b. Dead mouse or other varmint
c. Moisture (look for wet spots, or indications something is damp maybe in the walls, floors or even ceiling. Do you see any stains?
What ever it is get serious about finding it because if it is moisture it needs to be repaired pronto. A leak in the roof, walls or somewhere else can spell disaster. How about the water system? Any leaks there?
I wish you luck, and make sure you check back with us as you continue your hunt and find the source of the bad smell.