Start there, does it smell like sewage? mildew? old carpet? a dead mouse? new plastic?
Find the smell, then remove it. I always employ my wife for the "what does that smell like". I think women in general are better at that.
Last month I danced around for an evening trying to identify this strange smell around the electrical panel. It kinda smelt like burning plastic or rubber. When I hit panic and got her involved, she openned the back door and pointed to the vine in bloom and said, "dont they smell wonderful".