b top gun

Took a Nikon D780 out today for some video testing, had a Nikon 20mm f/1.8 prime mounted, shot some fast moving water through a narrow white water chute on a major river not far away. Camera set at 3840 x 2160 @ 24 frames per second. Two things happened on playback on my computer; first, I could see slight changes in the exposure (light to dark and back); exposure was not constant throughout the playback which was around 30 seconds long; second, I could hear some clicking in the audio even though the rushing water was making a good amount of noise. Any idea(s) what caused the exposure to change and what the source of the clicking sound might be? I have shot video with both a D7100 and a D850, used a couple of old Nikkor manual focus primes and both a Nikon 24-70 f/2.8 non VR and 70-200 f/2.8 current iteration on both the D7100 and D850; never noticed any fluctuation in exposure nor any audible clicking sounds. I am pretty sure I had the D780 set in full manual mode and manual focus.

b top gun

Did a check of the D780's movie menu and found that the AUTO ISO option had been set to "ON", turned it to "OFF". That same option on my D850's movie menu was also set to "OFF". That could account partly for the fluctuation in the brightness of the D780's video??? D850 is set to 1920 x 1080 @ 24 frames per second.

Bobspez Loc: Southern NJ, USA

Besides auto iso mentioned above, the clicking sounds like changes in aperture or auto focus on continuous autofocus seeking. Check it out indoors by pointing at a window and then at a dark corner, at near and far objects.

Chicago312 Loc: Western suburb, Chicago

Agree with the possibility that auto ISO could affect your change in exposure. I have similar issues when using my iPhone for video - changes in exposure, white balance, etc. Unless you use an app to manually control settings, sometimes things get messed up by the computer. Also, since you were shooting water outside, if you moved your focus point or if the amount of light reflecting back on the water changes, the auto iso could certainly affect your footage in an undesirable way.
Regarding the clicking noise, it could be anything - camera strap moving around, wildlife, etc. I would follow Bobspez’s advice and shoot indoors where you have control over noise.