My geuss is the smell is coming through a vent or from outside of a window. I would advise you to maybe turn any vents of, close any windows, maybe clean the vents. If the smell does not go away, this could also be caused by the drainage of houses nearby you, clogging up into your property, this is very unlikley but is possible.
If none of the above works I suggest you go and get a plumber of some kind, or someone who you think could help you with your problem.
