Does the computer power supply fan spin during start up? And should it always spin?

I have had recent noise coming from my computer, the sound is a fan that is making a squealing noise. I know it is not my video card because it has been recently replaced when the computer was making the sound. It is also not my misc. fans because I've disconnected them to diagnose. So that leaves my processor fan or my PSU fan. Now today I had the computer power shut off all of a sudden and a burning smell came from my computer. I disconnected it and left it for a while, then quickly plugged it in with the side door open to see if the fans were spinning. The processor fan is spinning, but the PSU one is not. I can't tell where the noise is coming from so just wondering if the PSU fan is suppose to spin all the time. That way I can replace the correct part. I don't have any replacement parts so I really need some help. Thanks in advance.|||If the CPU fan wasn't working, you'd be told in the setup when you boot your computer as it's essential that your CPU has a fan. If it's spinning, it sounds like it's fine. Your PSU on the other hand, is more than likely the problem. It should be spinning from the moment you power up. If it's not, it's got a problem and will likely need replacing.