Some things come to mind :

Plugs
Cables
Distributor
Rotor
Ignition coil

Does the problem come when the engine is warm and stays that way; or does it come and go intermittently?

By the way, the fuel system in our cars are pressurised so when you take off the gas cap, you will get the whoosh from the release of pressure in the fuel system.

I get the same thing with my car, too.

Cheers.