Help identify cloud-like artifacts on film

[mr.A]

Greetings,

not always, from time to time, I'm discovering these cloud-like artifacts on my 4x5 film shots. They are present on the film itself, its not a scanning artifacts. Artifacts are present on different film types(first photo is EKTAR100 color negative, second(rock) is EKTACHROME E100 slide). These 2 shots were done with different cameras and lens but with the same film holder(2 sided TOYO).

Cloud-like artifact pattern is always different on every frame that have them.

First color and second BW images are the same. 2nd is just one color channel with curves adjustments, to make artifacts more visible. On the BW image you can notice, that cloud-like artifacts have quiet the same silhouette like exposed part of the frame but with offset. Silhouette - I mean inner ledge for holding the film.

On the first photo, artifact structure on the right side of the frame is more shallow than on the rest of the frame.

Please, help with identifying the nature of this strange "noise" or "clouds".

Thank you in advance!