When running, the car is commanded to create a certain vacuum level within the fuel tank. When driving, the tank (and its fuel) heat up some. They say (after engine shutdown) the cars will go into a ECON mode (engine off natural vacuum) test. It says that pressure will continue to rise (in the tank) for a period of time, and it monitors that expected *pressure rise* that takes place (due to fuel heat up an no more vacuum being supplied). At some point it will start to cool down naturally.But during that rise period (after shutdown) they want to keep that pressure under control. So whenever its commanded, the charcoal canister *vent valve* will cycle to keep that pressure rise under control, until no longer required. That could last as long as 40 minutes (they say).
In all honesty, as much as I like my car, I don't hang around to see if any sounds will emanate from it after shutdown. So I can't say I've ever heard that valve doing its thing. But this is my best guess as what you are hearing.
