I just bumped my table a little (very slightly) and my computer was running. And then it started making this sound:
I was moving my ear a little in the case and it sounded like it was coming from the GPU, PSU or HDDs. I have AIO CPU cooling and the other fans are running fine.
I shutdown the PC for a minute and pulled out the electricity. I even took out the GPU power cable and with pressure air I cleaned the PSU a little.
Now after a few minutes the sound is gone.