Out of curiosity and because I'm too lazy to google:
I was reading the PZ test of the 18-55/2.8-4. There's not a single word about the behavior of the lens in terms of noise (which means "power-consumption"): As soon as I switch on the camera, I hear a little motor continuously spinning, no matter if OIS is ON or OFF. Okay, it's louder and more irregular, when OIS is ON, but I was expecting OIS OFF means "no power consumption". That's what I'm used to from the DSLR lenses.
This behavior is also shown by the 100-400, so the source can be the OIS or the electrical actuation of the aperture, which both lenses share in common.
Be it what it wants to be, but it sucks the battery empty quickly. Not as fast as a Sigma Merrill, but still, "spare-battery" became a must even for short walks.
My take is it the OS lens magnetically suspended by the stabilization system. My Nikon AF-S 70-300mm VR II has a fault and does not work in stand-by mode, when you focus and use half-press the VR system stabilizes nicely and all is sharp. When you let go the viewfinder image goes "clunk clunk" and the VF image drops, if you take a picture in this situation the image is de-centered and soft!
So the OS lens needs to be electrically suspended in a neutral position even when the OS is in the off position.
Modern giros have no motors but are minute crystal ceramic tuning forks that bend slightly when subjected to accelerations, this bending creates tiny electrical voltages which are fed to the electronic circuits/ electromagnets, the humming is the frequency of the OS system.
I'm not sure whether it is the IS or the AF.
IS / AF? Well, AF is set to Single, no Continuous. And the sound is also present, when I switch the lens to Manual - which at Fuji is focus by wire, so power consumption everywhere :unsure: IS sounds definitely different, to use a sound description: "remote blaster fire", so I hear the actuations if I press my ear close to the lens.
After some googling it appears that one of the last firmware updates brought the sound into the lens.
I think it requires power to hold the AF module in a defined position.
I can certainly confirm this for the Fuji 90mm f/2. If the power is off and you "shake" the lens and you will hear some rattling.
If you power on the rattling is gone in the same situation. And the 90 has no OIS.
I don't think so: 12 mm, 23 mm, 35 mm, 56 mm: all with focus drives, all silent when powered. But both OIS have also electrical aperture, but I tend to believe, the sound is an OIS on standby. I just don't get it so easily, when put on a tripod what good is it to use energy to keep something I don't need on standby?
The Pentax K-5 also "hums" in standby mode (the in-body IS).
If it's the AF I suppose that the IS group isn't mechanically locked thus it has to be actively kept in position.
FWIW, both the 16-50 and 50-140 also "rattle" if I remember this correctly.
The "rattling" because of an unpowered camera/AF is normal with a lot of other manufacturers using HSM/US AF motors or linear motors to drive the focusing unit. These units need to be moved with as less friction as possible and are not held in place when not powered because they don't have any "handbrake" or "parking position"
The sound I'm talking about is a single motor spinning, when camera is ON and the lens has OIS. I'm wondering what the purpose of this motor is to be "under steam" while doing nothing. Okay, nearly nothing: consumption of battery is also transformation of energy.