Odd Cabin Smell Issue

[Mafoo]

Not sure if this is much to go on but over the last couple of drives my Z has developed a foul exhaust/oily/maybe overheating smell.

The smell comes from the air heater vents. I cant seem to smell it when the bonnet is lifted. When I switch the aircon on it also goes. It seems worse when stationary and pulling off when the engine is hot.

I am guessing maybe an oil/exhaust leak but surely I would smell this outside of the car.

Has anybody ever experienced anything like this?

 

[mr nurburgring]

Could be loads of things, one thing to check is the rear of the rocker gasket, it weeps and drips a small amount of oil onto the cat/exhaust and causes a burning smell.