EWWWWWW!!!! I know this smell you speak of!
I bought a newly built condo in SoCal several years ago. A few months after moving in I noticed the "aroma" eminating from one of the walls. Apparently, they were drywalled in and starved to death...a whole family of the little buggers. Even after cutting a hole in the drywall to remove them, the odor persisted.
Try spreading some powdered lime on the area. IIRC, that's what I ended up putting on mine. It worked pretty quickly.
Randy