Hissing noise from fuel tank area after engine shut off

[Redroxz]

My 1990 300zx n/a with 75k miles has developed a hissing noise from the fuel tank area after the engine is shut off. It will gradually dissipate on it's own with time or immediately if I open the gas cap. It returns if I replace the gas cap. I see no fuel leaks and the car runs great. Anyone familiar with this?

[z.Leinbach]

Might be the fuel cap. How does the gasket around it look??

[Redroxz]

Thanks for the suggestion. I will pay closer attention to the cap. I suspect it may be the fuel check valve, but I am not sure if that is accessible from the top of the fuel tank or below.