My first guess would have been a tank-based fuel pump. Some cars the fuel pump will run to re-pressurize the fuel rail even if the engine is not running. If it is the fuel pump, and not noticed before, there may be some kind of problem with the fuel pressurization system. A bad fuel pressure regulator, etc.
My second guess would be as mentioned above, the evap system. It keeps the pressure in the gas tank at spec’d values, either a slight over-pressure, or a slight vacuum, varies car to car. So if this is the source there is a motorized pump which does this function. Again, if this seems like a new noise, and the source of the noise is the evap pump, there may be a problem in the evap system, some kind of leak or a bad purge valve maybe.
Other comments above are a possibility too.