Second opinion results: exhaust is in perfect working order. No leaks. Exhaust smell is possibly the result of being behind a car at a stop light (which is when I get a whiff).

Since this rumble (like a Subaru WRX/STI), is most prevalent when I am backing up or when I am stopped with the vehicle in drive and my foot on the brake, could this be an engine mount issue? Or am I just being sensitive to the sound because as a job I drive around in brand new Lincolns all the time?