EOS 5D Mark IV Black swooping line in image

[RB1809]

 I have a 5d mark iv which was purchased in 2018. I have never had the need to use use super slow shutter speeds until now. I’ve been shooting 1/8 and under. I’ve noticed this black swooping line appears in the bottom right/middle. It doesn’t stay in same spot. I’ve never had this problem when shooting faster shutter speeds from 1/60 and up. I first noticed it when using the sigma 14-24 lens and it also appears when using the sigma 50mm. I cleaned the camera which didn’t make a difference. Any help would be appreciated! 

[deebatman316]

Dust is more visible at smaller apertures ie (F/16) than larger ones ie (F/1.4). If its not visible at F/16 or another small aperture. Then its likely the shutter then. You can still look at the mirror with mirror lock up too. To see if dust is really there.