I drive an 88 Iroc z convertible with a TPI engine, automatic, stock. Runs great but occasionally when I pull in to my destination and turn off the car I get a hissing sound from underneath the car near the driver side rear tire, it lasts less than ten seconds... This does not happen all the time and there is no visible leaking and on very rare occasions I might get a fuel smell while driving.
Again the car runs just fine with no stumbling, idles right about 650rpm after warmup, mileage is still 19-21 around town. This hissing sound just appeared in the last six weeks or so and I am looking to insure it does not lead to anything big or worse yet a fire...Any help trying to diagnose this mystery would be appreciated.

Fivestar

 

Probably the white ufo vent thingy venting excess pressure out of the fuel tank.