So the fan in my PSU (Corsair CX600) makes a noise like the fan is catching on something whenever my PC is sitting idle (almost like a bad bearing). However, once I start using it, the noise no longer occurs. Any ideas as to what the problem might be?
It's possible the bearing is loose, but it's just as likely that the fan is vibrating against the case or plastic on the fan housing. I have this problem with one of my case fans, but I never really thought about it, since it usually goes away after a minute, and it's not every time I boot. What direction is your PSU fan pointed in, top or bottom?
Since there's a pattern to your problem, you should probably look into it. However, do not service your PSU. It's too dangerous. The capacitors need to be discharged, otherwise they still hold an extremely powerful charge that could seriously injure or kill you.
The bearing may indeed be bad... sometimes the pressure of airflow against the blades is enough to make the bearing behave normally again... personally I wouldn't worry too much about it until it's an issue at load (which it will be eventually if it's the bearing) if you rma something that isn't 'broken enough' you could end up getting it sent back to you.