[JJ_SO]

It's not disturbing, it's just asking for a pile more batteries...  :unsure:

[Klaus]

Well, the Olympus/Pentax in-body IS is also "always on".

[JJ_SO]

So far I thought, it's a motor for aperture activation. Sounds are different: focussing / OIS / AF and this basic noise.

[Klaus]

I guess it relates to the linear AF motor.

I'm not sure about the 100-400mm but the 90mm "rattles" when shaking. If you switch on the camera, the rattling is gone which probably means that the magnets are activated and the AF group is then "hovering" in its channel (which explains the fast AF).

 

That's my theory at least ;-)

[Brightcolours]

Hypothesis rather that theory

[JJ_SO]

A LEM (Linear Electromagnetic Motor) is silent when not in move. And even when in move, it happens close to noiseless. I wrote a lengthy post to point out two other suspects: the electrcally operated aperture blades and a gyroscopic element.But I think, only Fuji can confirm, and since I already have some little questions to the X-T2, I will include that one, too.

[JJ_SO]

This evening I tried to find something to learn a bot more about tracking with AF-C. I think, I should visit soccer games as well although I'm not interested in this sports, but thanks to a friendly duck who flew in and out I could get some shots. The shutter is addcitive, I admit. And the way the AF glues on an object - it's just me, but I never had such an experience with a DSLR.

 



 



 



 



 

And after all this action I forgot to dial back to AF-S, the evening sky was quite nice (and the AF-C capable)

 



 

No complaints so far for X-T2 and 100-400, although crop, although ISO 3200 and higher because it's no fast lens - but the JPGs out of camera look alright to me.