Jaguar vent smells

2001 S type Jag, 66000 miles, I have been getting a smell thru the a/c vents when I stop behind another auto especially. Sometimes if I am just sitting in the open with my car in park, sporadic, but very often I pull to a stop sign, or waiting in line, and here comes the smell, smells like exhaust. The muffler shop said no leaks, I have no drips in the garage for an oil leak, engine checks out fine so I am told. Any suggestions?

yes it is when the fresh intake is on, but had never had this smell before a month ago. I will check that filter. It also happens when I am parked by myself and leave the car running