My PSU started making some noises even when the fans were stopped. I also had USB problems (two USB devices burned) so I checked the PSU voltages with a multimeter.
I removed the PSU and connected it to a fan to see if it was on. I connected the green wire to a black wire and tested the voltages of the color cables on my 20 rails cable.
I noticed that my yellow (12V) rail was only showing 11.07V. Testing on the other yellow cables had the same result.
As this is bigger than the 5% margin of error I thought that I had a faulty PSU.
However after connecting the PSU to the motherboard, the values went up and my 12V rail went to 11.95.
Am I doing something wrong? Should I always test the PSU when it is connected to the PC?