My car had an exhaust leak that I could only smell when stopped. When driving the air coming through the rad would push the exhaust down the firewall and out so no smell. Exhaust when stopped would come up the firewall and seep into the intake vent for the heater. No smell on recirculate but this is a 2007 so recirculate shuts off outside air completely. Was somewhere by the manifold close to the engine as I could feel hot exhaust if I put my hand down there during a cool day. Not any nosier though. This was a 2.4 L 4 cylinder. Sold the car. In the old days we would put a potato up the exhaust and if the vehicle stalled then exhaust was sealed. It is funnier if you just put the potato up there and leave and start the car and rev it up. Just like a rocket blowing it out. ?