Dell Optiplex 5050 CPU fan juddering

Hello. The PC bios events show "Alert, the cover was previously removed". I have tried to use the PC for gaming, I bought an MSI GTX 1050 TI 4GT LP card, the main problem was that I wasn't able to use the GPU in the 16* slot, there was not enough place for the GPU. I used instead the 4* slot, the GPU worked. But in the end, I have bought a different PC for gaming. Please see this youtube video, once I turn off the PC from using the start/ turn off option, the CPU fan starts to spin at a high rpm, I have no idea why is this happening. This was an issue in the past, last year, but it happened very rarely. Now it happens very often. What do you suggest I do?