Very, very few people on the planet have ever seen a car powered by hydrogen fuel cells, much less ridden in or driven one.
But if you've wondered what they sound like in operation, now's your chance.
At a recent technology presentation, BMW showed off two different prototype vehicles powered by hydrogen fuel cells, part of its technology partnership with Toyota on advanced powertrains.
Now a recent video on the YouTube channel Cars demonstrates what a fuel-cell powered vehicle sounds like when it comes whooshing past you.
And given the mysterious, even ominous, flat-black paint on the heavily modified BMW i8 prototype car, the sounds somehow match: The vehicle sounds almost as unusual as it looks.
2016 Toyota Mirai hydrogen fuel-cell car, Newport Beach, CA, Nov 2014Enlarge Photo
As we learned from our drive in a pre-production 2016 Toyota Mirai hydrogen vehicle, the low-speed behavior is pretty similar to that of an electric car.
Both types of vehicles use powerful electric motors to turn the wheels; it's only the source of the electricity that's different.
ALSO SEE: 2016 Toyota Mirai: First Drive Of Hydrogen Fuel-Cell Sedan (Dec 2014)
At lower speeds, the Mirai sounds pretty much like a Prius hybrid in all-electric mode. Tire noise and a bit of background whine are the dominant sounds.
But at higher speeds, Toyota's hydrogen sedan adds a variety of additional noises as the compressors and pumps increase the volume of hydrogen fed to the fuel-cell stack to increase the power output.
BMW hydrogen fuel cell technologyEnlarge Photo
The "whoosh" you hear in the BMW video is matched by a similar sound, though less pronounced, in the Mirai. It comes from water vapor being expelled out the tailpipe.
The Toyota also manifested a high-pitched whir (we called it a "wheeze") from the air compressor that feeds oxygen into the fuel-cell stack.
No hydrogen fuel-cell car will generate anything like the powertrain noise of a vehicle powered by an internal-combustion engine which, after all, is producing thousands of tiny explosions to move itself along the road.
But we rather like this video because it shows off the alien nature of hydrogen fuel-cell powerplants.
Electric cars are quiet, perhaps with a little whine; hydrogen fuel-cell cars add an element of different, almost otherworldly, audio to the experience.