2003 cts Exhaust fumes in cabin

I was recently driving my cts wheh i noticed exhaust fumes coming in cabin at low speeds and idling. My wife said she heard a pop in engine and this is when the problem started.Took it to my mechanic,he found no exhaust leaks and no oil leaks,the car runs fine,and everything is in working order,but when i drive at slow speeds or idle at traffic lights,forget it car STINKS of exhaust fumes. Any help would be greatly appreciated. Thank You JB