I knew something was wrong with this old pc I put back together because it was too quiet. I found that one of the molex pins wasn't connected properly

I can assure you that the fan it was connected to is working now

Me when my pc doesn't sound like a jet engine: no that's wrong
@Roxy unrelated but I love the sound of the quantum CX series hard drives https://youtu.be/ElLgQPpWBxM
Quantum Fireball CX 13.6AT Hard Drive spin up/seek

YouTube