Question my cpu fan is randomly changing direction

[hahm]

every 25 seconds or so my cpu fan will randomly change direction this only started 2 days ago its is causing my pc to be loudly obnoxious

 

[rgd1101]

change direction? or stop and start?

 

[rubix_1011]

Are we certain this isn't the optical illusion created by rotational speed changes and the high-speed flicker of LED lighting? This is similar to why car wheels and helicopter rotors look like they spin one direction for a bit and then spin another, simply because their rotational speed increases or decreases with the frame refresh of the TV. Something similar happens when you view items in view of LED or florescent lighting (as they also have high-speed flicker).

The human eye cannot detect the flicker of this lighting outright, but when viewing something in movement that has continued motion (like a fan) you can detect the flicker and blade rotation because of how the object in motion reflects the flicker and the RPM change causing this observable difference.

https://io9.gizmodo.com/why-do-wheels-sometimes-appear-to-spin-backwards-1593807400