My daughter has a 2005 Impala with a 3.8 V-6. It has about 170,000 miles on it. She's had it for a little over a year. Tonight when she was driving home, she started noticing a REALLY strong fuel or exhaust smell coming into the cabin when she had the A/C on. She rolled down her windows and turned the A/C off. It got a little better and she was able to continue driving. I just checked it and I can smell it with the A/C on and I smell it under the hood. Doesn't seem to be loud like an exhaust leak, but I'm concerned and told her not to drive it in case it is some fuel issue. Don't want her becoming a fireball rolling down the road. My eyes are burning a little and I got a headache from the brief exposure I just had to it. In Las Vegas and it's mid 80's. Car does get parked outside. Haven't had rain in over 6 months so I doubt any mold issues.

 

You need to find out if it's gas or exhaust. The smells are distinct and so is the course of action.
As a point of reference for your nose, you can use the gas filler opening, or the tailpipe.

 

Found the issue. Once I pulled the engine cover off and started it up, the rear fuel line pressure connection was leaking fuel. Got a repair kit from Auto Zone for $20 and it's running nice and odor free...for now. Until the next issue that is.