94 Pontiac GrandAM petrol smell in AC

When I drive my 94 Pontiac GrandAM with the AC on, it runs fine and the AC air smells good. But after about 30 minutes driving with the AC
on, the AC air starts to smell like petrol. I tried to leave the rear window open a bit (like maybe it has to do with cabin pressure) to no avail. Actually, I'm not sure if it's petrol or oil that I'm smelling, but I don't want to take chances. But it's definitely hydrocarbon-based (but not burnt-oil-like).
I can bring it to my mechanic, but he'll have to drive it for half an hour before the smell appears. Anyone have a clue why this is happening? I can imagine a fuel leak or exhaust leak, but WHY is it appearing only AFTER 30 minutes later?
Summer's gonna be a scorcher. Help!
Shredrca Los Angeles, CA
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Check for a PCV hose off.
You could also arrange for the mechanic to check your car when you arrive at the appointed time, and drive it for at least 1/2 hour prior to arriving at your appointment.
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