When I first drove the Equinox, I thought that it had the quietness of a battery-powered electric car - or a hybrid in battery-only mode. But now that I spend more time with it, I see that's not really true. When you're rolling through a parking lot with the windows down, you can hear the fuel cell and its attendant hardware making buzzing and zapping noises. The noises are most evident when you shut the key off and get out of the car. Unlike a conventional car, shutting the key off does not immediately kill the engine. Instead it merely starts a shut-down procedure, in which the fuel source is slowly closed off, water is purged from the fuel-cell stack (to keep it from freezing in there during cold weather), excess electricity is fed to the high-voltage battery for storage, and the battery, if hot, is cooled by a fan that extracts air from the cabin. This whole procedure usually takes between thirty seconds and a couple of minutes, ending with a huff of water vapor exiting from the four (!) exhaust outlets. So when you're walking away from the car it's still making these buzzing and zapping noises and it's kind of weird.
Because the fuel cell is still using energy during its shut down procedure, the more fuel-efficient move is actually to leave it idling during brief stops rather than to turn it off and turn it back on again. That's quite the opposite of an internal combustion engine - one of the main sources of a hybrid's efficiency, for instance, is its ability to shut down the internal combustion engine when the car is just idling.